controls/preferences: add explanation of buffer underflow count, add reference in controls

Author: ronso0

source/chapters/appendix/mixxx_controls.rst

@@ -4407,7 +4407,8 @@ In the meantime, skins and controller mappings that still use them will keep wor
 
 .. mixxx:control:: [Master],audio_latency_overload_count
 
-   Counts buffer over and under-flows, maximum one per 500 ms.
+   Counts buffer over- and underflows, maximum one per 500 ms.
+   See :ref:`preferences-sound-other-options` for details.
    The count is displayed in Preferences -> Sound Hardware.
 
    :range: 0 .. n

source/chapters/preferences/sound_hardware.rst

@@ -155,6 +155,7 @@ macOS
 ^^^^^
 CoreAudio is the only Sound API on macOS.
 
+.. _preferences-sound-other-options:
 Other Sound Hardware options
 ----------------------------
 * **Multi-Soundcard Synchronization**: Mixxx is able to use two or more
@@ -177,12 +178,12 @@ Other Sound Hardware options
   in stereo because of speaker placement or playing in a space with lots of
   reverberation.
 
-* **Buffer Underflow Count**: Underflows (data is not available when needed)
-  indicate that some of the subsystems in Mixxx can't keep up with real-time
-  deadlines imposed by the current audio buffer size. This is useful to tune the
-  latency settings. If the counter increases, then increase your audio buffer
-  size, decrease the sample rate setting or change the sound API setting if
-  available.
+* **Buffer Underflow Count**: This counts overflows in output buffers or underflows
+  in input buffers, indicating either the output device(s) can't consume audio data
+  generated by the engine fast enough, or the input device(s) can't provide audio data
+  as fast as required by the current audio buffer size. If the counter increases,
+  you may try to increase your audio buffer size, decrease the sample rate setting,
+  change the sound API available or try another keylock/pitch-bend engine.
 
 Output and Input Devices
 ------------------------