\since for enums to not indicate "or modified in" if enum values have [since x.y]

I believe that now that there is support for documenting individual enum values as being introduced in a particular Qt version, the text for \since in an enum should no longer include "or modified in".

This may make older usage of \since for enums be incorrect, however, so if this behavior changes, then all usage of \since should also be examined and updated.

Or, alternatively, a special \since be introduced that does not emit the "or modified in".