API: read_excel signature
Replace the `**kwds` in `read_excel` with the actual list of supported keyword args. This doesn't
change any functionality, just nicer for interactive use.  Also a bit of clarification on the `thousands`
arg in the docstring.

Additionally, `chunksize` was a parameter in the `ExcelFile.parse` signature, but didn't do anything (xref #8011).  I removed this and raise `NotImplementedError` if passed, which is potentially breaking.
