Add docmentation for the framing header features

Author: Jake Champion

documentation/docs/globals/Request/Request.mdx

@@ -41,6 +41,14 @@ new Request(input, options)
     - `backend` _**Fastly-specific**_
     - `cacheOverride` _**Fastly-specific**_
     - `cacheKey` _**Fastly-specific**_
+    - `manualFramingHeaders`_: boolean_ _**optional**_ _**Fastly-specific**_
+      - : The default value is `false`, which means that the framing headers are automatically created based on the message body.
+        In "automatic" mode, a `Content-Length` is used when the size of the body can be determined before it is sent.
+        Requests sent in streaming mode, where headers are sent immediately but the content of the body is streamed later, will receive a `Transfer-Encoding: chunked` to accommodate the dynamic generation of the body.
+        In "manual" mode, any `Content-Length` or `Transfer-Encoding` headers will be honored.
+        You must ensure that those headers have correct values permitted by the [HTTP/1.1 specification](https://datatracker.ietf.org/doc/html/rfc7230#section-3.3.1).
+        If the provided headers are not permitted by the specification, the headers will revert to "automatic" mode and a diagnostic message will be logged about what was wrong.
+        If a `Content-Length` is permitted by the specification, but the value does not match the size of the actual body, the body will either be truncated (if it is too long), or the connection will be hung up early (if it is too short).
     - `fastly` _**Fastly-specific**_
       - `decompressGzip`_: boolean_ _**optional**_
         - Whether to automatically gzip decompress the Response or not.

documentation/docs/globals/Request/prototype/setManualFramingHeaders.mdx

@@ -0,0 +1,35 @@
+---
+hide_title: false
+hide_table_of_contents: false
+pagination_next: null
+pagination_prev: null
+---
+# Request.setManualFramingHeaders()
+
+The **`setManualFramingHeaders()`** method of the `Request` interface controls how the framing headers should be determined.
+
+By default the framing headers are set to "automatic" mode, which means they are created based on the body of the associated Request instance.
+In "automatic" mode, a `Content-Length` is used when the size of the body can be determined before it is sent.
+Requests sent in streaming mode, where headers are sent immediately but the content of the body is streamed later, will receive a `Transfer-Encoding: chunked` to accommodate the dynamic generation of the body.
+In "manual" mode, any `Content-Length` or `Transfer-Encoding` headers will be honored.
+You must ensure that those headers have correct values permitted by the [HTTP/1.1 specification](https://datatracker.ietf.org/doc/html/rfc7230#section-3.3.1).
+If the provided headers are not permitted by the specification, the headers will revert to "automatic" mode and a diagnostic message will be logged about what was wrong.
+If a `Content-Length` is permitted by the specification, but the value does not match the size of the actual body, the body will either be truncated (if it is too long), or the connection will be hung up early (if it is too short).
+
+## Syntax
+
+## Syntax
+
+```js
+setManualFramingHeaders(manual)
+```
+
+### Parameters
+
+- `manual` _: boolean_
+  - : Whether or not to use "manual" mode for the framing headers.
+
+### Return value
+
+`undefined`.
+

documentation/docs/globals/Response/Response.mdx

@@ -45,3 +45,11 @@ new Response(body, options)
       - : Any headers you want to add to your response, contained
         within a [`Headers`](../../globals/Headers/Headers.mdx) object or object literal of
         [`String`](../../globals/String/String.mdx) key/value pairs.
+    - `manualFramingHeaders`_: boolean_ _**optional**_ _**Fastly-specific**_
+      - : The default value is `false`, which means that the framing headers are automatically created based on the message body.
+        In "automatic" mode, a `Content-Length` is used when the size of the body can be determined before it is sent.
+        Responses sent in streaming mode, where headers are sent immediately but the content of the body is streamed later, will receive a `Transfer-Encoding: chunked` to accommodate the dynamic generation of the body.
+        In "manual" mode, any `Content-Length` or `Transfer-Encoding` headers will be honored.
+        You must ensure that those headers have correct values permitted by the [HTTP/1.1 specification](https://datatracker.ietf.org/doc/html/rfc7230#section-3.3.1).
+        If the provided headers are not permitted by the specification, the headers will revert to "automatic" mode and a diagnostic message will be logged about what was wrong.
+        If a `Content-Length` is permitted by the specification, but the value does not match the size of the actual body, the body will either be truncated (if it is too long), or the connection will be hung up early (if it is too short).

documentation/docs/globals/Response/setManualFramingHeaders.mdx

@@ -0,0 +1,35 @@
+---
+hide_title: false
+hide_table_of_contents: false
+pagination_next: null
+pagination_prev: null
+---
+# Response.setManualFramingHeaders()
+
+The **`setManualFramingHeaders()`** method of the `Response` interface controls how the framing headers should be determined.
+
+By default the framing headers are set to "automatic" mode, which means they are created based on the body of the associated Response instance.
+In "automatic" mode, a `Content-Length` is used when the size of the body can be determined before it is sent.
+Responses sent in streaming mode, where headers are sent immediately but the content of the body is streamed later, will receive a `Transfer-Encoding: chunked` to accommodate the dynamic generation of the body.
+In "manual" mode, any `Content-Length` or `Transfer-Encoding` headers will be honored.
+You must ensure that those headers have correct values permitted by the [HTTP/1.1 specification](https://datatracker.ietf.org/doc/html/rfc7230#section-3.3.1).
+If the provided headers are not permitted by the specification, the headers will revert to "automatic" mode and a diagnostic message will be logged about what was wrong.
+If a `Content-Length` is permitted by the specification, but the value does not match the size of the actual body, the body will either be truncated (if it is too long), or the connection will be hung up early (if it is too short).
+
+## Syntax
+
+## Syntax
+
+```js
+setManualFramingHeaders(manual)
+```
+
+### Parameters
+
+- `manual` _: boolean_
+  - : Whether or not to use "manual" mode for the framing headers.
+
+### Return value
+
+`undefined`.
+