Terminology proposal: "Maven module" -> "Maven sub-project"

[Martin Desruisseaux]

In order to avoid confusion between "Maven module" and "Java module", I 
suggest to update documentation for using "Maven sub-project" instead of 
"Maven module". However, the <module> XML elements in the POM would be 
unchanged for compatibility reason. There is apparently not so many 
places in the documentation that need to be updated. The ones that I 
found are part of pull request #1625:

https://github.com/apache/maven/pull/1625/commits/4bc46b4114396e6025645f3c5dae888f3d386981
   Martin