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
Merge pull request #4657 from jreback/bool

API: GH4633, bool(obj) behavior, raise on __nonzero__ always