Add more documentation to Table.hist
Add more documentation to Table.hist

Also makes `Table.hist` throw a more helpful error message when the
Table contains invalid values (`np.flexible`, which includes strings).
Before, the method raised an error about not being able to operate on
"flexible types", which is not extremely helpful without knowledge of
numpy types.

ericz82/datascience,data-8/datascience,dsten/datascience,ericz82/datascience