make documentation of octave tracking match spkr(4) and mml.py

MMLFILE.rst

@@ -44,6 +44,9 @@ Symbols of MML
     disables it; with octave tracking enabled, jumps between notes are
     limited to 6 half-tones, otherwise, the octave is incremented or
     decremented accordingly. Octave tracking is disabled by default.
+    Octave tracking is suspended for one note after an octave change,
+    including setting the default octave at the beginning of the track,
+    and only applies to letter notes.
 
 :P(n):
     Pause. n may range from 1-64; the current L value is used if omitted.