I found that the testAnnotationProcessor dependency configuration seems to be missing from the docs at The Java Plugin (and any other page at docs.gradle.org except for the API docs of the Java Plugin which gives the “null documentation” that TEST_ANNOTATION_PROCESSOR_CONFIGURATION_NAME is “The name of the test annotation processor configuration.”)
Also I am not sure about how the ...Elements configurations fit into the picture (for example for contributions to/from other configurations), because these are undocumented, too: apiElements, runtimeElements, sourcesElements
And what is your question?
This is a community-place where mainly users help users.
If you want to report a bug, you need to open an issue over at GitHub.
Besides that your statements are all incorrect.
testAnnotationProcessoris annotated on the Java Plugin page at The Java Plugin. It is _sourceSet_AnnotationProcessor and the sourceSet in that case is test which is added for the test JVM test suite added by the JVM test suite plugin.
And also the apiElements and sourceElements configurations are not undocumented. They are not documented on the Java plugin page, but most probably - even if it adds them - because they are consumable configurations and typically a project meant for consumption should apply the Java Library plugin instead and there those configurations are documented: The Java Library Plugin
The sourceElements configuration also is documented, at the description of withSourcesJar() which adds and wires it properly: The Java Plugin