From Joomla! Documentation

H:h stands for help header. It is a system of templates that render (help) headers in the various namespaces for pages on Joomla! Documentation. For details see Project:hatnotes.

The behaviour is configurable per namespace, e.g. Template:H:h Help displays links to the Mediawiki handbooks parts and optionally a sidebar Template:xyz. Usage: {{h:h|xyz}}

H:h Template displays this text because it belongs to the Template namespace and adds H:h templates to Category:Template documentation. Its counterpart is H:f Template.