Restrict tags for HiddenTextExpander and Truncate

[khiga8]

What
This PR restricts tags for HiddenTextExpander to :span, and also restricts tags for Truncate to [:div, :span, :p]. I chose these tags based on current usage in dotcom and what seemed reasonable.

This PR also makes explicit the tag argument for Markdown so consumers know they can set the tag argument. I was unsure what the allowed tags for this component should be based on current usage of article, tdin dotcom.test_renders_different_tags` also seemed to imply that this component can be relatively flexible so I did not add restrictions. Open to thoughts if you have any.

Why
Part of #491

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[joelhawksley]

@manuelpuyol you wrote test_renders_different_tags. Did you intend to imply that any tag could be passed for Markdown?

[manuelpuyol]

@manuelpuyol you wrote test_renders_different_tags. Did you intend to imply that any tag could be passed for Markdown?

Looking at dotcom usage for markdown-body we also use it with td and article. The test was intended to guarantee that we could use those tags
I think that restricting that component to :div, :td, :article is fine!

[manuelpuyol]

mind updating this changelog to be after # main?