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: NJosh 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.comSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/parse-options.h

@@ -152,6 +152,9 @@ struct option {
 /* parse_options() will filter out the processed options and leave the
  * non-option argments in argv[].
  * Returns the number of arguments left in argv[].
+ *
+ * NOTE: parse_options() and parse_options_subcommand() may call exit() in the
+ * case of an error (or for 'special' options like --list-cmds or --list-opts).
  */
 extern int parse_options(int argc, const char **argv,
                          const struct option *options,