Clarify that nx and nb is not allowed

[ahankinson]

If a note in the key signature is sharpened, but the flat value is desired, it may be a practice that nb is used to first cancel the sharp, and then flatten. e.g.,

@xFC ... nbC

The specs should clarify that this is NOT permitted, and that simply bC is sufficient (that is, the accidentals give an absolute value, not a value relative to the key signature.)