Improve documentation (javadoc) for the UserScriptService

The UserScriptService (documented in https://extensions.xwiki.org/xwiki/bin/view/Extension/User%20Module/API/ ) has interesting methods for manipulating user profiles, read or write.

However, the javadoc of the class is very brief, and does not properly disambiguate methods.

For example, getProperties() and getAllProperties() seem to have exactly the same javadoc, while there should be some explanation of the difference between them.