VFS: note error returns in documentation for various lookup functions

VFS: note error returns in documentation for various lookup functions

Darrick recently noted that try_lookup_noperm() is documented as
"Look up a dentry by name in the dcache, returning NULL if it does not
currently exist." but it can in fact return an error.

So update the documentation for that and related functions.

Link: https://lore.kernel.org/all/20260218234917.GA6490@frogsfrogsfrogs/
Cc: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: NeilBrown <neil@brown.name>
Link: https://patch.msgid.link/20260224222542.3458677-2-neilb@ownmail.net
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>