Explain the Receiver.close() method in the user guide

Signed-off-by: Sahas Subramanian <[email protected]>

src/frequenz/channels/_receiver.py

@@ -123,6 +123,17 @@
 # If we get here, the receiver was stopped
 ```
 
+# Closing receivers
+
+To deal with situations where data from a particular stream is no-longer required,
+receivers offer a [`close()`][frequenz.channels.Receiver.close] method that can be used
+to stop it from receiving new messages.
+
+After `close()` is called, we can still receive messages that are in the receiver's
+buffer, but as soon as the receiver's buffer has been drained, trying to receive further
+messages from a *closed* receiver will raise a
+[`ReceiverStoppedError`][frequenz.channels.ReceiverStoppedError].
+
 # Advanced Usage
 
 !!! Warning inline end