Document hex(blob) function

Fixes #3975

docs/sql/functions/blob.md

@@ -16,6 +16,7 @@ This section describes functions and operators for examining and manipulating [`
 | [`blob || blob`](#blob--blob) | `BLOB` concatenation. |
 | [`decode(blob)`](#decodeblob) | Converts `blob` to `VARCHAR`. Fails if `blob` is not valid UTF-8. |
 | [`encode(string)`](#encodestring) | Converts the `string` to `BLOB`. Converts UTF-8 characters into literal encoding. |
+| [`hex(blob)`](#hexblob) | Converts `blob` to `VARCHAR` using hexadecimal encoding. |
 | [`octet_length(blob)`](#octet_lengthblob) | Number of bytes in `blob`. |
 | [`read_blob(source)`](#read_blobsource) | Returns the content from `source` (a filename, a list of filenames, or a glob pattern) as a `BLOB`. See the [`read_blob` guide]({% link docs/guides/file_formats/read_file.md %}#read_blob) for more details. |
 
@@ -45,6 +46,14 @@ This section describes functions and operators for examining and manipulating [`
 | **Example** | `encode('my_string_with_ü')` |
 | **Result** | `my_string_with_\xC3\xBC` |
 
+#### `hex(blob)`
+
+<div class="nostroke_table"></div>
+
+| **Description** | Converts `blob` to `VARCHAR` using hexadecimal encoding. |
+| **Example** | `hex('\xAA\xBB'::BLOB)` |
+| **Result** | `AABB` |
+
 #### `octet_length(blob)`
 
 <div class="nostroke_table"></div>