DOC: Update excel read dependency on ``xlrd``

Since 98f65b19f918b825ae6cb1001cd8e91f45377cc7,
``xlrd`` handles reading both ``.xls`` and ``.xlsx`` Excel Files.
DOC: Update docstrings to reflect benefit from resolving #4131

1bd8f57d68686f268922a1e230c5a8f2e4e3cc9d resolves #4131 without
updating docstrings in relevant functions.  No way to know that keep_default_na
and verbose are valid keyword arguments without inspecting the parsers.py code
and documentation
Merge pull request #4154 from davidshinn/improve_excel_docs

DOC: Improve excel docs