docs: don't warn about starlark_doc_extract output

[alexeagle]

The community already relies on this protobuf format, and that it is a default output of the starlark_doc_extract rule.

Are there specific breaking changes you imagine making here?

FYI @meisterT

[tetromino]

Hmm. We do our best to evolve the structure of the output (the proto message format) in a backwards-compatible way. However, the content of the output is absolutely not stable; just to pick the most obvious example, the docstrings for builtin rule/macro attributes will change whenever they are updated in Bazel's source tree.

So the exact output of starlark_doc_extract from a given .bzl file may change even with minor version bumps in Bazel.

Is there a better way of expressing this idea?

[tetromino]

@alexeagle , what do you think about the following wording to replace the old warning:

Note: the exact output of this rule - in particular, the set of natively-defined common rule attributes and their docstrings - can change even with minor Bazel releases. For this reason, we suggest that any golden tests based on this rule to be tagged as "manual" and to require a specific Bazel version when run in CI.

[alexeagle]

Sure, I think it's pretty obvious that the documentation strings aren't constant as changes land to the code being documented. want to update the PR?

[tetromino]

@alexeagle - yes, please

[alexeagle]

Done, thanks!

[tetromino]

Thanks, sent for review internally