man: Fix FUTEX_WAIT/FUTEX_WAKE explanation

The description of the futex_wait parameters is incorrect when
explaining the futex value.  Correct it and rewrite the rest of the
futex operations description to make them clearer.

Closes: https://github.com/axboe/liburing/issues/1625
Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>
Link: https://patch.msgid.link/20260831154426.115944-2-krisman@suse.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>