Add doc control decorators to the api-generator.

There are two primary goals here.

First: This gives us a starting point to cleanup/remove the "private_map" and
"do_not_descend" map from `generate_lib.py`. Replacing them with local decorators.
The primary reason for these in the first place is to deal with unsealed modules (The docs generator will crawl anything it can reach).

Second: This gives us options for splitting documentation output between "Users" and "Subclass Implementers". There are many "public" functions that you do not need to know about unless you are interested in implementing a sub-class. These only make sense to document on the base class "Layer", "Model", "Metric".
PiperOrigin-RevId: 208701899