doc: Improve message protocol

* Add missing details to some of the fields.
* Removed Cam-service specific implementation details.
* Fixed grammar.
* Re-worded some of the paragraphs.

Signed-off-by: Filipe Rinaldi <filipe.rinaldi@arm.com>
Signed-off-by: Vineeth Raveendran <vineeth.raveendran@arm.com>