Update the documentation for seqplay usage

src/mockingboard/seqplay.pla

@@ -844,6 +844,12 @@ musicPlay(trackPtr, trackRepeat)
     Params:
         Pointer to a track sequence created from the cvtmidi.py tool
         Repeat flag - TRUE or FALSE.
+    The first time its is called, it will try and search for a MockingBoard.
+    However, it is noted that this can cause problems if a Z-80 card is installed.
+    The scanning routine might cause a hang if it encounters a Z-80 card before
+    it finds a MockingBoard. In order to make this robust, it might be best to
+    prompt the user to search for the MockingBoard, enter the actual MockingBoard
+    slot, or skip the MockingBoard and use the internal speaker.
 
 musicStop()
     Stop playing a track sequence in the getUpperKey routine
@@ -872,3 +878,20 @@ spkrPWM(samples, speed, len)
         Pointer to 8 bit pulse width samples
         Speed to play through samples
         Length of sample
+
+The main routines for sequencing music are:
+
+mbSequence(yield, func)
+spkrSequence(yield, func)
+noSequence(yield, func)
+
+    All three try and provide more functionality than would be present in
+    previous music sequencers. The MockingBoard sequencer will attempt to play up
+    to 9 tones per sound generator (18 if a MockingBoard II is found). Up to
+    four notes will be played simultaneously on the internal speaker. In order
+    to play more notes than the hardware normally supports, a technique using
+    arpeggio (playing multiple notes in a quick sequence rather than concurrently)
+    pulls off this feat. The sequencers will immediately return if a keypress is
+    detected. Finally, during the sequencing, a background function can be periodically
+    called every 'yield' time which has a resolution of a 16th of a second. Pass
+    in zero for 'yield' and/or 'func' to disable any background calls.