DOCS: add documentation for --volume-gain options

DOCS/interface-changes.rst

@@ -27,6 +27,7 @@ Interface changes
 ::
 
  --- mpv 0.38.0 ---
+    - add `--volume-gain`, `--volume-gain-min`, and `--volume-gain-max` options
     - add `--secondary-sub-ass-override` option
     - remove shared-script-properties (user-data is a replacement)
     - add `--secondary-sub-delay`, decouple secondary subtitles from

DOCS/man/options.rst

@@ -2002,6 +2002,13 @@ Audio
 
     Since mpv 0.18.1, this always controls the internal mixer (aka "softvol").
 
+``--volume-gain=<db>``
+    Set the volume gain in dB. This is applied on top of other volume and gain
+    settings.
+
+``--volume-gain-max=<0.0-150.0>``, ``--volume-gain-min=<-150.0-0.0>``
+    Set the volume gain range in dB (default: -96 dB min, 12 dB max).
+
 ``--replaygain=<no|track|album>``
     Adjust volume gain according to replaygain values stored in the file
     metadata. With ``--replaygain=no`` (the default), perform no adjustment.