Document approach to styling

[lognaturel]

• Body can't contain arbitrary markup as in W3C XForms
• Certain body elements can contain markup (e.g. labels)
• Markdown and a subset of HTML are supported

Reference implementation: https://github.com/getodk/collect/blob/master/collect_app/src/main/java/org/odk/collect/android/utilities/HtmlUtils.java#L23

Clients should either render the styling or strip it out.

[eyelidlessness]

To capture/distill some of my thinking from the Slack discussion:

• This feels like it belongs in the spec, even if optional, at least to specify the opt-out behavior. Current examples of opt-out behavior are not ideal: producing the Markdown, including the HTML subset, as-is (e.g. **bold** _italics_ <br> -> **bold** _italics_ <br>). Removing formatting would be preferable if/when Markdown is unsupported.
• Inclusion in the spec doesn't have to/shouldn't mean writing yet another Markdown standard; ideally we can map the current reference implementation's behavior to pertinent aspects of CommonMark (which is a rare counterexample of the aforementioned yet-another-spec problem).
• Having an explicit derived spec subset, our implementations can hopefully defer to/become oriented around existing CommonMark implementations. Collect could still be treated as the reference implementation, but as a reference implementation it might look more like configuration to achieve the Markdown/HTML subset we specify.