player/command: deprecate ao-volume and ao-mute

These properties were mostly only useful before the removal of
--softvol=no. At present, mpv has its own volume mixer and never touches
the audio server volume at all. There's no reason for mpv to provide
these properties, if users wish to change the audio server volume, it's
trivial to do so in a Lua/Javascript script.

Moreover, ao-volume presents the audio server volume as-is, meaning
without any conversion or normalization. As such, the meaning of the
value changes depending on the audio server in use or the configuration
of the audio server. There's no correct way for mpv to do this
normalization. As such, this kind of functionality is better suited for
a Lua/Javascript script where the user can normalize the values to their
preferred scale, and not bloat mpv with pointless properties that are
confusing and not useful.

Author: llyyr

DOCS/interface-changes/deprecate-ao-volume-mute.txt

@@ -0,0 +1,2 @@
+deprecate `ao-volume`
+deprecate `ao-mute`

DOCS/man/input.rst

@@ -2702,10 +2702,14 @@ Property list
     on a linear curve, while with PulseAudio this controls per-application volume
     on a cubic curve.
 
+    .. warning:: This property is deprecated and will be removed in the future.
+
 ``ao-mute`` (RW)
     Similar to ``ao-volume``, but controls the mute state. May be unimplemented
     even if ``ao-volume`` works.
 
+     .. warning:: This property is deprecated and will be removed in the future.
+
 ``audio-params``
     Audio format as output by the audio decoder.
     This has a number of sub-properties: