BUG: TimedeltaArray+Period, PeriodArray-TimedeltaArray
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

motivated by the FIXME comment in test_timedelta64.py
Can you explain what this PR is changing? can you also add an example in the docs (maybe in the timedelta section) > Can you explain what this PR is changing?

PeriodIndex - TimedeltaIndex always raises in master, but for some `freq` combinations should not.  This fixes that. thanks