parse-options: format argh like error messages

"Keep it homogeneous across the repository" is in general a
guideline that can be used to converge to a good practice, but
we can be a bit more prescriptive in this case.  Just like the
messages we give die(_("...")) are formatted without the final
full stop and without the initial capitalization, most of the
argument help text are already formatted that way, and we want
to encourage that as the house style.

Noticed-by: ZheNing Hu <adlternative@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

parse-options.h

@@ -82,9 +82,9 @@ typedef enum parse_opt_result parse_opt_ll_cb(struct parse_opt_ctx_t *ctx,
  *   stores pointers to the values to be filled.
  *
  * `argh`::
- *   token to explain the kind of argument this option wants. Keep it
- *   homogeneous across the repository. Should be wrapped by N_() for
- *   translation.
+ *   token to explain the kind of argument this option wants. Does not
+ *   begin in capital letter, and does not end with a full stop.
+ *   Should be wrapped by N_() for translation.
  *
  * `help`::
  *   the short help associated to what the option does.