Doc: edit texteditor plugin API docs

Fix grammar and style issues and punctuation.
Use the \a command instead of \param (not a QDoc command).

Change-Id: Ib048f0219a2c857e5fc3fcd858afbcbd58819193
Reviewed-by: David Schulz <david.schulz@digia.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>
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>