State the comment bar in CONTRIBUTING

[tony]

Follow-up to #8, which corrected 172 comments against the policy in AGENTS.md. The contributor guidance did not describe the bar those changes were held to.

It read:

Comments explain why, not what. The surrounding code is the reference for density and idiom.

Both halves misdirect. "Why, not what" is not absolute — AGENTS.md itself keeps a sketch of what a dense algorithm does when the local operations do not reveal the whole. And pointing at the surrounding code as the reference for density is circular when the surrounding code is what needed correcting: the sweep found blocks running six to ten lines against a stated one-to-two line ceiling, so a contributor matching what they saw would have reproduced the defect.

Neither half says when to delete, which is the decision most often being made.

What replaces it

• the bar a comment has to clear, in terms of what the code, types, assertions and tests do not already carry
• the ceiling, and what it means when a comment exceeds it
• the categories that are always deleted — narration, restatement, excuses, hand-tracked values — and the tiebreak toward deletion
• that XML documentation is judged the other way round, by caller value, and that CS1591 makes omitting it a build error in the published projects
• a link to AGENTS.md, which carries the full policy and its three gates

Documentation only. No code changes.