DOC: clarified to_excel and read_excel (GH6260)
as discussed in issue #6260
- io.rst: added a paragraph explaining the difference in behaviour from 0.12
- excel.py: added docstring for has_index_names

thanks to jmcnamara

ps: sorry about the pollution with multiple commits, I'm still learning how to rebase/squash

Thanks for the contribution! I just added some small comments.

There also seems something wrong with your history of commits (undo and redo again). You can clean this up by rebasing interactively (https://github.com/pydata/pandas/wiki/Using-Git#wiki-fetch-and-then-rebase-interactively-to-squash-reword-fix-otherwise-change-some-commits) and then squash the 5 commits in one.
 @jorisvandenbossche  thanks for the education. Please let me know if everything is correct now.
 That looks very good now! Thanks again
