fix #254: credman alignment: update #getAssertion section a la PR #498 (

#665)

* actually improve #254, and fix #661

* DiscoFrmExtSource(options) -> (origin, options)

* make [[DiscoFrmExtSource]]'s exposition match [[Create]]'s

* deal with yet another fix #254 straggler in [[Create]]

* get rid of |global| in [[DiscoFrmExtSource]]

* remove 'in parallel' and 'global' stuff from #discover-from-external-source alg

* work on #discover-from-external-source alg to improve #254

* finish (one hopes) work on #discover-from-external-source alg to fix #254

* minor editorial

* repair #createCredential intro parag, improves issue #671

* complete fix #671