DOC: Surface / doc mangle_dupe_cols in read_excel
xref https://github.com/pandas-dev/pandas/issues/10523#issuecomment-438465073.

We already support `mangle_dupe_cols` (in the same vein as CSV).  It was just hidden in `**kwds`.  This just documents it in `read_excel` (it behaves exactly as it does in `read_csv`) and tests that we inform the end user that `mangle_dupe_cols=False` is not supported...yet.

Explicitly surfacing relevant CSV-related arguments will allow us to eventually remove the magical `**kwds` and obviate need for validation (xref #22189).
