DOCS/mpv: the playback position is not remembered after poweroff

This doesn't actually work on either Windows or Linux with --terminal.
With --no-terminal or --no-input-terminal the SIGTERM handler is never
registered, so it definitely can't work.

Just remove the note about signals because it would be complicated to
explain that they don't terminate abruptly only with --terminal and only
if that signal has a handler, and it wouldn't be of interest to most
users.