unshare.2: Update pointer to in-kernel unshare documentation

unshare.2: Update pointer to in-kernel unshare documentation

Linux commit v4.12-rc1~137^2~33^2 converted unshare documentation
(previously resided in Documentation/unshare.txt) to ReStructured
format, along with relocating it to
Documentation/userspace-api/unshare.rst.

* man2/unshare.2 (.SH SEE ALSO): Amend pointer to
  Documentation/unshare.txt with change introduced in Linux 4.14
  (move to Documentation/userspace_api/unshare.rst).

Signed-off-by: Eugene Syromyatnikov <evgsyr@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>