Doc: edit projectexplorer API docs
Fix grammar, punctuation, and style issues. Use standard wording for
QDoc commands. Remove \brief commands from function descriptions.
Move some function descriptions directly above the functions, so
that the \fn command can be removed.
Change-Id: Iedf4f0041af24541a982241f99bd4906e86af916
Reviewed-by:
Daniel Teske <daniel.teske@digia.com>