argp: Improve comments.

* argp/argp-help.c: Add sectioning comments. Write NULL to designate a
null pointer.
(struct hol_entry): Fix comment regarding sort order of group.
(hol_entry_short_iterate, hol_entry_long_iterate): Add comment.
(until_short, canon_doc_option, hol_entry_qcmp): Improve comment.
(hol_cluster_is_child, argp_hol): Move functions.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>