landlock: Document FS access right for pathname UNIX sockets

landlock: Document FS access right for pathname UNIX sockets

Add LANDLOCK_ACCESS_FS_RESOLVE_UNIX to the example code, and explain it
in the section about previous limitations.  The bulk of the interesting
flag documentation lives in the kernel header and is included in the
Sphinx rendering.

Cc: Justin Suess <utilityemal77@gmail.com>
Cc: Mickaël Salaün <mic@digikod.net>
Signed-off-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20260327164838.38231-13-gnoack3000@gmail.com
Signed-off-by: Mickaël Salaün <mic@digikod.net>