Rename method.

It's a description of the arity so let's call it that.
Refactor: make caller pass a signature object.

This clears the way to use a slightly different
signature implementation (e.g. for blocks).
Reword docstrings.

I'm about to introduce a new `description` method
and having docstrings say "it is described precisely"
when not referring to the description is confusing.
Use method signature verifier for `and_yield` block verification.