Document approach to styling

* 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.

Start with the styling requirements in this issue and the referenced collect_app/src/main/java/org/odk/collect/android/utilities/HtmlUtils.java implementation. Document which Markdown and HTML styling is supported, including body and label content, and specify that clients must render or strip unsupported styling.

Written by the indexing model from the issue text.