Docstrings for top-level constant values (strings, lists, dicts, structs)

We generate API docs for bazel build language objects which include a doc field (such as rules, providers, and aspects) or have a docstring set by convention (modules, functions); but we do not document any other Starlark values such as strings, lists, tuples, dicts, or structs.

However, .bzl files frequently define string/list/etc. constants as part of their API, and documenting them would seem a good idea.

Prior art in Python land: https://www.sphinx-doc.org/en/master/usage/extensions/autodoc.html#directive-autoattribute