Update to AI-based tooling policy (#6477)

The goal is to clarify that tool-generated submissions are fine, but
emphasize the requirements we have on the operators of these tools. The
inspiration for the two aspects emphasized comes from the discussion
around an update to LLVM's policy in
https://github.com/llvm/llvm-project/pull/154441, and in Fedora's
policy:

https://docs.fedoraproject.org/en-US/council/policy/ai-contribution-policy/

I've not used those policies _exactly_, as I think we may want somewhat
simpler and less formal guidance, but the goal is to remain
directionally aligned.

That said, I'm not attached to the current iteration of the wording, it
still feels a bit excessively formal or wordy to me. Suggestions on
wording improvements very welcome in addition to thoughts and feedback
on the overall direction.

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>
Co-authored-by: Dana Jansens <danakj@orodu.net>
Co-authored-by: Carbon Infra Bot <carbon-external-infra@google.com>

CONTRIBUTING.md

@@ -232,21 +232,44 @@ as well as helpful tooling that will ease the contribution process. For example,
 #### Using AI-based contribution tools
 
 All submissions to Carbon need to follow our
-[Contributor License Agreement (CLA)](#contributor-license-agreements-clas), in
-which contributors agree that their contribution is an original work of
-authorship. This doesn’t prohibit the use of coding assistance tools, but what’s
-submitted does need to be a contributor’s original creation.
-
-All contributions, regardless of what tools are used, are also still subject to
-normal code review and our
-[guidelines and standards](#contribution-guidelines-and-standards) below.
-
-Additionally, we want contributions to Carbon to also be viable as contributions
-to LLVM so that we can move things between these projects where relevant. We
-selected our license in part for this reason, and the same should be true for
-the use of AI-based coding tools. Any contributions to Carbon should also abide
-by the guidance in the
-[LLVM Developer Policy around AI generated code](https://llvm.org/docs/DeveloperPolicy.html#ai-generated-contributions).
+[Contributor License Agreement (CLA)](#contributor-license-agreements-clas),
+which covers any original work of authorship included in the submission. This
+doesn't prohibit the use of coding assistance tools, including tool-, AI-, or
+machine-generated code, as long as these submissions abide by the CLA's
+requirements.
+
+All contributions, regardless of what tools are used, are also still the
+responsibility of the operator of these tools and subject to normal code review
+and our [guidelines and standards](#contribution-guidelines-and-standards)
+below. We also emphasize two additional requirements for contributors operating
+or using AI-based tools:
+
+1. **Contributions should not become extractive of the project and community**:
+   the value added should outweigh the overhead of landing the contribution. The
+   overhead of landing contributions ranges from code review, to discussions,
+   distractions from the current project priorities, or growing maintenance
+   burden without growing maintainers.
+
+2. **Each PR should be transparent about the tooling used** in proportion to how
+   much of the PR was produced by the tool and whether the tool is a standard
+   one for the project. For example, formatting with the standard tools is
+   reasonable to assume without further comment. But if a PR is largely derived
+   from running a specific Python script, regular expression, or AI-based tool
+   over the codebase, we ask that its commit message is transparent about this
+   and include a description of how the tool was used to formulate the change.
+   For PRs largely derived from AI-based tooling, we suggest following the
+   pattern established by the Fedora Project to mark commits with
+   `Assisted-by: ...`.
+
+Our policies and practices here are inspired by and aim to be roughly compatible
+with several other open source projects:
+
+-   [Fedora Project's policy](https://docs.fedoraproject.org/en-US/council/policy/ai-contribution-policy/)
+-   [LLVM Developer Policy around AI generated code](https://llvm.org/docs/DeveloperPolicy.html#ai-generated-contributions)
+
+As the open source community evolves and learns how best to integrate these
+tools into project and development workflows, we expect to reflect that with
+updates and improvements here.
 
 ### Contribution guidelines and standards