Improve documentation for dynamic classes.

An attempt to improve the clarity of the documentation for using verifying doubles with dynamic classes.
@JonRowe's right -- a `class_double` can use `respond_to?` to check for a method's existence but an `instance_double` can't.  Dynamic methods handled by `method_missing` are supported on `class_double` as long as `respond_to?` is implemented properly (e.g. via `respond_to_missing?`, but on an `instance_double`, RSpec has no object to call `respond_to?` on -- instead it uses `klass.method_defined?`, which does not includes messages handled by `method_missing`. (How could it?)