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 and jorisvandenbossche
Merge pull request #6302 from iled/doc

DOC: clarified to_excel and read_excel (GH6260)