Add unit test and documentation for module API

Add unit test and documentation for module API

Related to commit ae6c7a44394d4a5f943f1fc92f4de015ec7bcc96.
Previously, only property access on module API was tested.
This commit adds a test which calls a method on a module API.
It also adds documentation for the qmlRegisterModuleApi functions.

Task-number: QMLNG-33
Reviewed-by: Martin Jones