Missing content in cffi.verify documentation

Created originally on Bitbucket by ncoghlan (Nick Coghlan)

Reviewing the docs based on the python-dev discussion, I noticed most of the parameters to cffi.verify are not documented along with the function - they are only documented with the Verifier constructor signature.

It would be better to document the parameters fully in the convenience function documentation, and then only document the differences in the lower level Verifier docs.