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

[tetromino]

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

[aaronsky]

did a little bit of research for this just now. not super familiar with the code, but i don't see anything in the proto that would give us this information. so maybe this requires changes to bazel first?

[tetromino]

@aaronsky - yes.

[tetromino]

I've started working on this. Requires Starlark parser changes, which means this won't be available until Bazel 9.

Current idea is to use Sphinx-style #: doc comments, which will need to be propagated from lexer to parser and then to global variables in compiled Module objects.