updated keycodes media descriptions

Platform compatibility descriptions added for mute, volume up/down

docs/keycodes.md

@@ -152,12 +152,12 @@ When defining a [keymap](keymap.md) each key needs a valid key definition. This
 |`KC_WWW_REFRESH`|`KC_WREF`||
 |`KC_WWW_FAVORITES`|`KC_WFAV`||
 |`KC_STOP`||Stop|
-|`KC__MUTE`||Mute|
-|`KC__VOLUP`||Volume Up|
-|`KC__VOLDOWN`||Volume Down|
-|`KC_AUDIO_MUTE`|`KC_MUTE`||
-|`KC_AUDIO_VOL_UP`|`KC_VOLU`||
-|`KC_AUDIO_VOL_DOWN`|`KC_VOLD`||
+|`KC__MUTE`||Mute (macOS)|
+|`KC__VOLUP`||Volume Up (macOS)|
+|`KC__VOLDOWN`||Volume Down (macOS)|
+|`KC_AUDIO_MUTE`|`KC_MUTE`|Mute (Windows/macOS/Linux)|
+|`KC_AUDIO_VOL_UP`|`KC_VOLU`|Volume Up (Windows/macOS/Linux)|
+|`KC_AUDIO_VOL_DOWN`|`KC_VOLD`|Volume Down (Windows/macOS/Linux)|
 |`KC_MEDIA_NEXT_TRACK`|`KC_MNXT`|Next Track (Windows)|
 |`KC_MEDIA_PREV_TRACK`|`KC_MPRV`|Previous Track (Windows)|
 |`KC_MEDIA_FAST_FORWARD`|`KC_MFFD`|Next Track (macOS)|