With the changes in JDK-8219801 and JDK-8220379 in the JDK 13 javadoc tool, all HTML heading tags in class documents must start with <h2> and be nested correctly (i.e., no skipping levels). We have a few places where we are using <h1> tags and several more where we are using <h3>. As a result, the build of FX docs via `gradle javadoc` fails with JDK 13.

This needs to be done before we upgrade our boot JDK to 13 as part of JDK-8232064, since the JDK 13 javadoc tool will enforce this restriction. The changes are compatible with older JDK versions (e.g., 11 or 12) so this should be done before we do the upgrade of the boot JDK.