Documentation: ioctl: add entry for nsfs.h

Documentation: ioctl: add entry for nsfs.h

All userspace ioctls major/magic number should be documented in
Documentation/userspace-api/ioctl/ioctl-number.rst, so add
the entry for <linux/nsfs.h>.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Andrey Vagin <avagin@openvz.org>
Cc: Serge Hallyn <serge@hallyn.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: linux-doc@vger.kernel.org
Cc: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20210210182248.18101-1-rdunlap@infradead.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>