manpage: update keybindings
vf_vapoursynth: print more diagnostics on error
vaapi: fix uninitialized value read

Found with valgrind. This is somewhat terrifying, because the VA-API API
function is supposed to fill these values, and we access them only if
the API functions return success. So this shouldn't have happened.
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.
Release 0.5.0