API Docs: Clarify BLooper::PostMessage and BMessenger::SendMessage

... docs to make it clear which methods work synchronously and which work
asynchronously. A small number of related edits are included as well as a
couple of pedantic whitespace changes. Clarify sync vs. async reply
handling better in BMessenger class description. Add The to make sentences.

Change-Id: I3069934fc5e82dda25331e85884d6d0c0c100dfd
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2178
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

Small fixes to BMessenger docs

Move the operator function docs over too

Add BMessenger docs

Small fixes to BMessenger docs

Move the operator function docs over too

Add BMessenger docs