[docs] Communication subsystem docs update for MDL-78551

[safatshahin]

Some more API's added as a part of MDL-78551, adding docs for them.

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[sarjona]

Hi @safatshahin!
Thanks for improving the communication API docs to include changes implemented in https://tracker.moodle.org/browse/MDL-78551
As you'll see, I left a few points that should be clarified before merging this patch. TIA!

[safatshahin]

Hi @sarjona
Thank you for your comments. I have resolved all of them and made the changes.
Please have a look now.
Cheers!

[sarjona]

HI @safatshahin!
Thanks for reviewing and considering my comments :-)
I'm happy to approve and merge this :-)