document set_default_verify_paths caveats (#667)
document set_default_verify_paths caveats (#667)

* document set_default_verify_paths caveats

fixes #642

* add a bit more detail

* weasel words

reaperhulk/pyopenssl,pyca/pyopenssl,hynek/pyopenssl,reaperhulk/pyopenssl,hynek/pyopenssl