vfs: improve i_op->atomic_open() documentation

Fix documentation of ->atomic_open() and related functions: finish_open()
and finish_no_open().  Also add details that seem to be unclear and a
source of bugs (some of which are fixed in the following series).

Cc-ing maintainers of all filesystems implementing ->atomic_open().

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Sage Weil <sage@inktank.com>
Cc: Steve French <sfrench@samba.org>
Cc: Steven Whitehouse <swhiteho@redhat.com>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>