Revert b5b7436..49f51c8

This rolls back to commit b5b743643fd6f4e34e06ce57b55d12363a028198.
DOC: clarified to_excel and read_excel

as discussed in issue GH6260
. io.rst: added a paragraph explaining the difference in behaviour from
0.12
. excel.py: added docstring for has_index_names
thanks to jmcnamara
cleared docstring, fixed indentation

pull GH6302
thanks to jorisvandenbossche
Merge branch 'doc' of https://github.com/iled/pandas into doc