Improve the _merge module description

Explain how to use the `merge()` function and the `Merger` type on a
higher level than the function documentation.

This module is not publicly available, so users won't be able to access
the information in IDEs for example, but it will be rendered as part of
the User Guide.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

src/frequenz/channels/_merge.py

@@ -1,7 +1,51 @@
 # License: MIT
 # Copyright © 2022 Frequenz Energy-as-a-Service GmbH
 
-"""Merge messages coming from channels into a single stream."""
+"""Merge messages coming from multiple receivers into a single receiver.
+
+# Usage
+
+If you just need to receive the same type of messages but from multiple sources in one
+stream, you can use [`merge()`][frequenz.channels.merge] to create a new receiver that
+will receive messages from all the given receivers:
+
+```python
+from frequenz.channels import Anycast, Receiver, merge
+
+channel1: Anycast[int] = Anycast(name="channel1")
+channel2: Anycast[int] = Anycast(name="channel2")
+receiver1 = channel1.new_receiver()
+receiver2 = channel2.new_receiver()
+
+async for value in merge(receiver1, receiver2):
+    print(value)
+```
+
+If the first message comes from `channel2` and the second message from `channel1`, the
+first message will be received immediately, and the second message will be received as
+soon as it is available.
+
+This can be helpful when you just need to receive messages and don't care about
+where are they coming from specifically. If you need to know where the message came
+from, you can use [`select()`][frequenz.channels.select] instead.
+
+# Stopping
+
+A merge receiver will be stopped automatically when all the receivers that it merges are
+stopped. When using the async iterator interface, this means that the iterator will stop
+as soon as all the receivers are stopped. When using
+[`receive()`][frequenz.channels.Receiver.receive], this means that the method will raise
+a [`ReceiverStoppedError`][frequenz.channels.ReceiverStoppedError] exception as soon as
+all the receivers are stopped.
+
+If you want to stop a merge receiver manually, you can use the
+[`stop()`][frequenz.channels.Merger.stop] method.
+
+When using [`receive()`][frequenz.channels.Receiver.receive], you should make sure to
+either stop all the receivers that you are merging, or to stop the merge receiver
+manually. This is to make sure that all the tasks created by the merge receiver are
+cleaned up properly.
+"""
 
 from __future__ import annotations