API: add ddof to expanding/rolling_cov()

Closes #8279 
closes #8064
Correct. (Technically ``ddof`` isn't an explicit parameter of ``expanding/rolling_var/std()``, but is accepted as part of ``kwargs`` and passed on to ``algos.roll_var()``.)

I will add to the release note a comment about ``expanding/rolling_var/std`` support for ``ddof``.