All plugins contain an XML plugin descriptor file. It is located in the plugin jar file under the name META-INF/rhq-plugin.xml.

The actual XML Schema is annotated with documentation and can be used in your IDE to help with auto-completion tips. The XML Schema .xsd file can be found here: http://git.fedorahosted.org/git/?p=rhq/rhq.git;a=blob;f=modules/core/client-api/src/main/resources/rhq-plugin.xsd;hb=master