Remove '\p', '\c' and '\e' doxygen markup from doxy, as it should

Remove '\p', '\c' and '\e' doxygen markup from doxy, as it should
improve plain text doxy readability.

See the thread: "[RFC] Should we use doxygen markup?".

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@19122 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b