video: don't keep multiple pointers to hwdec info struct

This makes a certain corner case simpler at a later point.
input.conf: make explanatory text more readable

Or at leats this is the intention. It's a bit hard to tell which
information is needed, and which not.
input.conf: add some bindings for changing audio-delay

Requested on: https://github.com/mpv-player/mpv/commit/90ec3334174e80c16f00971886223a3afabc1aca#commitcomment-7331673

Might remove or remap them again later.