BUG: vectorized DateOffset match non-vectorized
closes #11370 

Also added a bit of documentation on the semantics for anchored offsets

currently blocked by #11406 - either need to change the vectorized or non-vectorized implementation of `QuarterBegin`  As discussed in that issue, my opinion is the vectorized version is the "right" (or at least consistent) one.

@chris-b1 can you rebase / update
 @jreback - can you take another look at #11406?  Based on that I still need to change `QuarterBegin` for either the vectorized or non-vectorized case.
 #11777 merged! pls update
 @jreback - updated
 lgtm! thanks!
