man: document io-wq handling of file system managed locks

The kernel side no longer fails lock requests on files with their own
->flock()/->lock() handling (NFS, FUSE, etc) with -EOPNOTSUPP, instead
they are handled via the io_uring async worker pool. Update the
io_uring_prep_flock and io_uring_prep_ofd_lock man pages accordingly:
document the worker occupancy caveat for contended waits, replace the
-EOPNOTSUPP error with -EINTR for signal-interrupted waits, and scope
the "no deadlock detection" statement to generic VFS locks, as a file
system may implement its own.

Signed-off-by: Jens Axboe <axboe@kernel.dk>