Remove some mentions to channels in sender and receiver

Receivers are not necessarily attached to a channel, as we already have
some utility receivers that generates messages from other sources (like
time, files or arbitrary events).

For senders we don't have that yet, but there is also no reason to limit
them to channels.

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

Author: llucax

src/frequenz/channels/_receiver.py

@@ -199,7 +199,7 @@ def __aiter__(self) -> Self:
         return self
 
     async def receive(self) -> _T_co:
-        """Receive a message from the channel.
+        """Receive a message.
 
         Returns:
             The received message.

src/frequenz/channels/_sender.py

@@ -62,7 +62,7 @@ class Sender(ABC, Generic[_T_contra]):
 
     @abstractmethod
     async def send(self, msg: _T_contra) -> None:
-        """Send a message to the channel.
+        """Send a message.
 
         Args:
             msg: The message to be sent.