doc: mention more functions added in cygwin 1.7.0

doc: mention more functions added in cygwin 1.7.0

* doc/posix-functions/mbsnrtowcs.texi: Mention recent cygwin 1.7.0
addition.
* doc/posix-functions/open_wmemstream.texi: Likewise.
* doc/posix-functions/wcsnlen.texi: Likewise.
* doc/posix-functions/wcsnrtombs.texi: Likewise.
* doc/posix-functions/wcstod.texi: Likewise.
* doc/posix-functions/wcstof.texi: Likewise.
* doc/posix-functions/wcstoimax.texi: Likewise.
* doc/posix-functions/wcstok.texi: Likewise.
* doc/posix-functions/wcstoumax.texi: Likewise.

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