missing_safety_doc: guide them out of an HTML block with a header

[notriddle]

changelog: [missing_safety_doc]: suggest fixes for HTML
changelog: [missing_errors_doc]: suggest fixes for HTML
changelog: [missing_panics_doc]: suggest fixes for HTML

Fixes #15353

When there's a missing header, but code that looks like one in an HTML block, show a suggestion to add the missing blank line.

[rustbot]

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[notriddle]

r? rust-lang/clippy

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.