futex man: add note on using linked timeouts

The io_uring wait syscalls do not support passing in a timeout in
the request itself, like the sync futex syscalls do. Instead, it is
encouraged to use linked timeouts to achieve the same thing.

While in there, also fixup some .B$ tags that should've been .BR.

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