Allow to dispatch getting documentation on objects. (#13975) Base for #13860, so that object can be queried for documentation on their fields/properties. Typically this allows the following, to extend the doc documentation when requesting information on a field. ```python class DictLike: def __getitem__(self, k): if k.startswith('f'): return "documentation for k" else: raise KeyError class Bar: __custom_documentations__ = DictLike() faz = 1 @property def foo(self): return 1 b = Bar() b.faz? ```