options: actually deprecate --mute=auto

Also, make it internally actually an alias to "no".

DOCS/interface-changes.rst

@@ -72,6 +72,7 @@ Interface changes
       treating it as a hardware overlay (without applying GL filtering). Also
       to be changed in 0.22.0: the --fs flag will be reset to "no" by default
       (like on the other platforms).
+    - deprecate --mute=auto (informally has been since 0.18.1)
     - deprecate "resume" and "suspend" IPC commands. They will be completely
       removed in 0.22.0.
     - deprecate mp.suspend(), mp.resume(), mp.resume_all() Lua scripting

DOCS/man/input.rst

@@ -2114,6 +2114,9 @@ caveats with some properties (due to historical reasons):
     When set as option, the maximum (set by ``--volume-max``) is not checked,
     while when set as property, the maximum is enforced.
 
+``mute``
+    The option has a deprecated ``auto`` value, which is equal to ``no``.
+
 ``playlist``
     The property is read-only and returns the current internal playlist. The
     option is for loading playlist during command line parsing. For client API

DOCS/man/options.rst

@@ -1134,9 +1134,10 @@ Audio
     Audio delay in seconds (positive or negative float value). Positive values
     delay the audio, and negative values delay the video.
 
-``--mute=<auto|yes|no>``
-    Set startup audio mute status. ``auto`` (default) will not change the mute
-    status.
+``--mute=<yes|no|auto>``
+    Set startup audio mute status (default: no).
+
+    ``auto`` is a deprecated possible value that is equivalent to ``no``.
 
     See also: ``--volume``.
 

options/options.c

@@ -514,8 +514,8 @@ const m_option_t mp_opts[] = {
     // values <0 for volume and mute are legacy and ignored
     OPT_FLOATRANGE("volume", softvol_volume, 0, -1, 1000),
     OPT_CHOICE("mute", softvol_mute, 0,
-               ({"auto", -1},
-                {"no", 0},
+               ({"no", 0},
+                {"auto", 0},
                 {"yes", 1})),
     OPT_CHOICE("gapless-audio", gapless_audio, 0,
                ({"no", 0},