DOC: indexing with alignment
Per request of issue #13950, added documentation clarifying alignment of DataFrame Objects and assignment of subset of one DataFrame to subset indices of another Dataframe.

Main issue was that if indices did not match, behavior of Dataframe would render rows of non-matched indices as NaN.  This could be corrected by assigning a non-Dataframe object of same appropriate dimension and value configuration to the proper Dataframe indices.


- [ ] closes #xxxx
 - [ ] tests added / passed
 - [ ] passes ``git diff upstream/master | flake8 --diff``
 - [ ] whatsnew entry
