Tweak documentation to cover cygwin argz bugs.

Tweak documentation to cover cygwin argz bugs.

* m4/argz.m4 (gl_FUNC_ARGZ): Mention date of last known cygwin
argz bug fix; no code change needed since no cygwin releases
occurred between the last fix and the bug being tested.
* doc/glibc-functions/argz_add.texi (argz_add): Document the argz
module and recently fixed cygwin bugs.
* doc/glibc-functions/argz_add_sep.texi (argz_add_sep): Likewise.
* doc/glibc-functions/argz_append.texi (argz_append): Likewise.
* doc/glibc-functions/argz_count.texi (argz_count): Likewise.
* doc/glibc-functions/argz_create.texi (argz_create): Likewise.
* doc/glibc-functions/argz_create_sep.texi (argz_create_sep):
Likewise.
* doc/glibc-functions/argz_delete.texi (argz_delete): Likewise.
* doc/glibc-functions/argz_extract.texi (argz_extract): Likewise.
* doc/glibc-functions/argz_insert.texi (argz_insert): Likewise.
* doc/glibc-functions/argz_next.texi (argz_next): Likewise.
* doc/glibc-functions/argz_replace.texi (argz_replace): Likewise.
* doc/glibc-functions/argz_stringify.texi (argz_stringify):
Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>