manpage: update config file locations

Also add some explanations how the config paths are determined.
sub: fix undefined behavior with dvd://

The string could get reallocated.

CC: @mpv-player/stable
af_volume: fix calculations including replay-gain

rgain is not an additive value. It's a multiplier/gain.

Previous behaviour produced negative level values in some cases
(when rgain < 1.0) which caused volume to be louder when its value
was lowered.

CC: @mpv-player/stable

Signed-off-by: Mohammad Alsaleh <CE.Mohammad.AlSaleh@gmail.com>
Signed-off-by: wm4 <wm4@nowhere>