perf tools: Document the fact that parse_options*() may exit

Generally, calling exit() from a library is bad practice.  Eventually
these functions might be redesigned so that they don't exit.  For now,
just document the fact that they do.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/97b1af06cc3b18dd0f49e655d6d659eaa64ecde5.1450193761.git.jpoimboe@redhat.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>