Doc: add rudimentary Module item reference

Change-Id: I763486128efbc6e49e3bdb4d563444cee0851ffc
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
introduce Module.validate

This property can be used to validate the property values of a module.

Task-number: QBS-319
Change-Id: I3d50c95cca16c5f0ad356fca02a0588679444cc9
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>