manual: Complete @standards in argp.texi.

	* manual/argp.texi (ARGP_HELP_USAGE): Add missing header and
	standard annotation.
	(ARGP_HELP_SHORT_USAGE): Likewise.
	(ARGP_HELP_SEE): Likewise.
	(ARGP_HELP_LONG): Likewise.
	(ARGP_HELP_PRE_DOC): Likewise.
	(ARGP_HELP_POST_DOC): Likewise.
	(ARGP_HELP_DOC): Likewise.
	(ARGP_HELP_BUG_ADDR): Likewise.
	(ARGP_HELP_LONG_ONLY): Likewise.
	(ARGP_HELP_EXIT_ERR): Likewise.
	(ARGP_HELP_EXIT_OK): Likewise.
	(ARGP_HELP_STD_ERR): Likewise.
	(ARGP_HELP_STD_USAGE): Likewise.
	(ARGP_HELP_STD_HELP): Likewise.