These variables aren't really magic and they typically have been documented in wiki pages. Unfortunately they are sometimes too much of a technical detail to make it into the official documentation and the wiki pages have been marked as "obsolete" as they are not actively maintained by Alfresco staff and may partially fall out-of-date as new versions are release.
The fieldHtmlId is defined in the form.lib.ftl just before the field control template FTL is included when building a form.
