Clarify that using channels is not enforced

The SDK doesn't enforce the use of channels, but it's the recommended
way to communicate between actors and the one used by SDK actors.

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

Author: llucax

src/frequenz/sdk/actor/__init__.py

@@ -32,8 +32,8 @@
 the traditional actor programming model but also has some unique features:
 
 - **Message Passing:** Like traditional actors, our Actor class communicates
-  through message passing. To do that they use [channels][frequenz.channels]
-  for communication.
+  through message passing. Even when no particular message passing mechanism is
+  enforced, the SDK uses [Frequenz Channels][frequenz.channels] for communication.
 
 - **Automatic Restart:** If an unhandled exception occurs in an actor's logic
   (`_run` method), the actor will be automatically restarted. This ensures