Update doc review section of maintainer guide

bazelbuild#321 has been merged.

docs/maintainers_guide.md

@@ -40,11 +40,13 @@ widespread pain, and shouldn't be done lightly. Therefore:
 
 In addition, make sure that new code is documented and tested.
 
-If a PR adds or changes any docstrings, check that Markdown docs in `docs`
-directory are updated; if not, ask the PR author to run
-`./docs/regenerate_docs.sh`. (See
-https://github.com/bazelbuild/bazel-skylib/pull/321 for the proposal to automate
-this.)
+If a PR changes any docstring in an existing module, the corresponding
+`stardoc_with_diff_test` in `docs` will fail. To fix the test, ask the PR
+author to run `bazel run //docs:update`.
+
+If a PR adds a new module, make sure that the PR also adds a corresponding
+`stardoc_with_diff_test` target in `docs/BUILD` and a corresponding `*doc.md`
+file under `docs` (generated by `bazel run //docs:update`).
 
 ## Making a New Release