Merge pull request #4585 from jreback/boolean_comp

BUG: Fix boolean comparison with a DataFrame on the lhs, and a list/tuple on the rhs GH4576
TST: test for GH4633, bool(obj) behavior

BUG: GH4633, rever to GH1073, whereby __nonzero__ always raises for all NDFrame objects
DOC: docs to add detail on gotchas for true value testing