FR: Add support for documenting Starlark-defined transitions

Starlark-defined transitions are defined with the `transition` function, which does not have a `doc` parameter. The FR is to add that to Bazel and support it in stardoc.

Start by reading Bazel's Starlark transition documentation and Stardoc's handling of the `transition` function. Done means the function accepts a `doc` parameter and Stardoc includes that documentation in generated output, with the relevant behavior verified by the project's tests.

Written by the indexing model from the issue text.