Documentation: atomic_open called with shared lock on non-O_CREAT open

The exclusive lock is only held when O_CREAT is set.
Signed-off-by: NJeff Layton <jlayton@kernel.org>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/filesystems/locking.rst

@@ -105,7 +105,7 @@ getattr:	no
 listxattr:	no
 fiemap:		no
 update_time:	no
-atomic_open:	exclusive
+atomic_open:	shared (exclusive if O_CREAT is set in open flags)
 tmpfile:	no
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