Be more mathematicly precise about definition of recurrence [NFC]

This clarifies the interface of the matchSimpleRecurrence helper introduced in 8020be0b for non-commutative operators.  After ebd3aeba, I realized the original way I framed the routine was inconsistent.  For shifts, we only matched the the LHS form, but for sub we matched both and the caller wanted that information.  So, instead, we now consistently match both forms for non-commutative operators and the caller becomes responsible for filtering if needed.  I tried to put a clear warning in the header because I suspect the RHS form of e.g. a sub recurrence is non-obvious for most folks.  (It was for me.)