Clarify documentation around module.name and module.dir

From the reference at https://maven.apache.org/archetype/archetype-models/archetype-descriptor/archetype-descriptor.html#class_module is is unclear how the attribute name is evaluated. For me the following behaviour would make sense

• dir: the name of the source directory (from which the new module will be created)
• name: the name of the module directory (destination). Usually is equivalent to artifactId. Currently the name does not seem to be evaluated at all.

That way no placeholder replacement on the weird syntax _<property>_ would need to take place on the dir entry, but rather that would just specify from where to copy the files for the module (this can always be hardcoded). The placeholder mechanism (i.e. the regular one with the $ syntax would then only be necessary on id and name attributes.