Wording change: Don't use asserts for validating argument types (#518)

* Wording change: Drop arg type assertions for methods defined by WebIDL

These don't improve readability of the spec.

To keep this commit focused, it does not touch similar assertions
for internal algorithms.

(Also, convert one NOTE into an ISSUE since it represents pending
spec work.)

For #455

* Wording change: Convert algorithm type assertions into parameter types

Improve internal algorithm readability by including the type of
parameters in the declaration of the algorithm, rather than as
assertions.

This commit just touches the algorithms with assertions. Other
algorithms could be improved with parameter type declarations, but
that'll be a follow-up.

For #455

Author: inexorabletash