doxygen: Fix doxygen errors

This patch fixes a number of errors and warning messages in the doxygen
log. Specifically, it addresses:
* A number of files incorrectly places a '\brief' tag immediately after
  a '\file' tag. Doing so emits a warning, as '\file' takes an optional
  argument specifying which file the doxygen comment is for. As '\brief'
  is not a file, doxygen was unamused.
* A grouping of Stasis Topics and Messages in rtp_engine.h was
  incorrectly terminated. We now correctly terminate the grouping, which
  prevents members of rtp_engine.h from showing up in the wrong group.
* Group indicators which are not part of the Stasis Topics and Messages
  group were removed. Group indicators without an \addtogroup or
  \ingroup have no meaning.

Change-Id: Ia1415ffec6767e27233ae1cae5ed5970de5656d4