Merge: no_warning: modules without a module clause do not cause `missing-doc`

Rationale: the presence of a `module` clause is a good heuristic to
discriminate quick and dirty prototypes from nice and clean modules

Pull-Request: #1078
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Romain Chanoir <chanoir.romain@courrier.uqam.ca>