Doc: replace \a with \c in .qdoc files

The \a command should only be used in code documentation
(in .cpp files), where QDoc uses it to check that all
formal parameters are documented and emits warnings if
they are not or if they are misspelled.

Change-Id: I37710afcd860f9baa3042337aca98fac6701b997
Reviewed-by: Niels Weber <niels.weber@digia.com>