Clarify equals method docstring
Make explicit that equals method requires that columns have the same dtypes but not that indices have the same types (e.g. `pd.DataFrame({1:[0], 0:[1]}).equals(pd.DataFrame({1.0:[0], 0.0:[1]}))` returns `True` while `pd.DataFrame({1:[0], 0:[1]}, dtype='float32').equals(pd.DataFrame({1:[0], 0:[1]}, dtype='float64'))` returns `False`)

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