Fix format of numpydoc parameters section

See section 4, "Parameters", of
https://github.com/numpy/numpy/blob/master/doc/HOWTO_DOCUMENT.rst.txt

When a parameter can only assume one of several values, put them in braces,
with the default first, and if the parameter is optional, indicate that.
Merge pull request #392 from adamrp/fix_table_numpydoc

Fix format of numpydoc parameters section