DOC: clarify that ExcelFile is not deprecated but only moved (#5435)
closes #5435. Clarified that ExcelFile is not deprecated but just moved. I don't use Excel myself, so can somebody check if it makes sense what I wrote? (@

For now, I didn't add ExcelFile to api.rst (but ExcelFile.parse is already in there, so it is already in some way mentioned), as I am not sure if all it's methods (`mro`?) should be public (this has possibly to be cleaned up first)
