fs/namei.c: update kerneldoc of atomic_open()

fs/namei.c: update kerneldoc of atomic_open()

The comments above atomic_open() contain several errors:

  - atomic_open() does not return 0 if successful
  - @path is not updated

Fix those and be more explicit about when FMODE_OPENED and FMODE_CREATED
are set. Change to a full kerneldoc.

Signed-off-by: Jori Koolstra <jkoolstra@xs4all.nl>
Link: https://patch.msgid.link/20260710164233.827744-4-jkoolstra@xs4all.nl
Reviewed-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>