libtraceevent: Man pages for tep plugins APIs

Create man pages for libtraceevent APIs:

  tep_load_plugins(),
  tep_unload_plugin()

Signed-off-by: Tzvetomir Stoyanov <tstoyanov@vmware.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: linux-trace-devel@vger.kernel.org
Link: http://lore.kernel.org/linux-trace-devel/20190903133434.30417-1-tz.stoyanov@gmail.com
Link: http://lore.kernel.org/lkml/20190919212542.216189588@goodmis.org


Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>