Best way to extend a type doc string

General Usage
6
  1. a docstring is registered in the module where it is defined.
  2. only one docstring in a module is associated with a (function/type, signature) pair
    a. defining another one replaces the incumbent.
  3. a docstring for a bare type has a null signature (i.e., Union{})
    a. constructors can add new docstrings.