Improve the _sender module description

Give a short introduction about how `Sender`s are created and used,
including how to handle errors.

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/_sender.py

@@ -1,7 +1,53 @@
 # License: MIT
 # Copyright © 2022 Frequenz Energy-as-a-Service GmbH
 
-"""Channel sender and associated exceptions."""
+"""Sender interface and related exceptions.
+
+# Senders
+
+Messages are sent to a [channel](/user-guide/channels) through
+[Sender][frequenz.channels.Sender] objects. [Senders][frequenz.channels.Sender] are
+usually created by calling `channel.new_sender()`, and are a very simple abstraction
+that only provides a single [`send()`][frequenz.channels.Sender.send] method:
+
+```python
+from frequenz.channels import Anycast
+
+channel = Anycast[int](name="test-channel")
+sender = channel.new_sender()
+
+await sender.send("Hello, world!")
+```
+
+Although [`send()`][frequenz.channels.Sender.send] is an asynchronous method, some
+channels may implement it in a synchronous, non-blocking way. For example, buffered
+channels that drop messages when the buffer is full could guarantee that
+[`send()`][frequenz.channels.Sender.send] never blocks. However, please keep in mind
+that the [asyncio][] event loop could give control to another task at any time,
+effectively making the [`send()`][frequenz.channels.Sender.send] method blocking.
+
+# Error Handling
+
+!!! Tip inline end
+
+    For more information about handling errors, please refer to the
+    [Error Handling](/user-guide/error-handling/) section of the user guide.
+
+If there is any failure sending a message,
+a [SenderError][frequenz.channels.SenderError] exception is raised.
+
+```python
+from frequenz.channels import Anycast
+
+channel = Anycast[int](name="test-channel")
+sender = channel.new_sender()
+
+try:
+    await sender.send("Hello, world!")
+except SenderError as error:
+    print(f"Error sending message: {error}")
+```
+"""
 
 from abc import ABC, abstractmethod
 from typing import Generic, TypeVar
@@ -12,7 +58,7 @@
 
 
 class Sender(ABC, Generic[_T]):
-    """A channel Sender."""
+    """An endpoint to sends messages."""
 
     @abstractmethod
     async def send(self, msg: _T) -> None: