Documentation: filesystems api-summary: add namespace.c

Add fs/namespace.c to the filesystems api-summary docbook.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Link: https://lore.kernel.org/r/20210318025227.4162-2-rdunlap@infradead.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>