Release descriptions without paragraph/ol/ul tags should fail the validator

[Dretch]

Currently this gets past the validator without any warnings or errors, but is apparently not valid (one consequence is that the description won't show in flathub - flathub/flathub#5304):

<release version="0.0.15.5" date="2024-05-19">
  <description>
    Upgrade to latest Stackage LTS, simplify desktop file insertion.
  </description>
</release>

Note that when a top-level <description> is missing the <p> tag then the validator fails with app-description-required. So I suppose the same validation should be done for <description> tags inside <release> tags.

In addition, the <description> documentation at https://www.freedesktop.org/software/appstream/docs/sect-Metadata-Releases.html#tag-release-description and https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-description could more clearly state that HTML tags (at least <p>) are required - currently it just says that are supported.