docs: document recently fixed glibc printf bug

docs: document recently fixed glibc printf bug

Document it as a known bug, but one where we don't provide a
workaround since programmers are unlikely to hit it in practice.

* doc/posix-functions/fprintf.texi (fprintf): Document it.
* doc/posix-functions/printf.texi (printf): Likewise.
* doc/posix-functions/vfprintf.texi (vfprintf): Likewise.
* doc/posix-functions/vprintf.texi (vprintf): Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>