docs: document tag types for insertHtmlContent

[homotechsual]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

Clarifies currently undocumented behaviour of the tag types.

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[slorber]

👍 thanks