Doc: edit coreplugin docs

Remove \brief commands for functions.
Use standard wording for QDoc commands.
Fix style and grammar issues.
Use the \a command instead of the \c command for attributes
in function descriptions.
Do not use \returns, because it does not exist.

Change-Id: Icd32b519670cb376e246bab3a58fe7e98d2529ea
Reviewed-by: Eike Ziller <eike.ziller@digia.com>