Clarify rule two

Rule 2 currently says:

> _Non-breaking change_. Otherwise, if only new bindings, types, classes, non-orphan instances or modules (but see below) were added to the interface, then A.B **MAY** remain the same but the new C **MUST** be greater than the old C. Note that modifying imports or depending on a newer version of another package may cause extra non-orphan instances to be exported and thus force a minor version change.

There's an ambiguity here. It could be read as requiring C to be increased only when A.B remains the same (which I believe to be intended), or it could be read as requiring C to be increased regardless of whether A.B remains the same. Could this wording be made more clear?