IdConflictPolicy: improve documentation for CREATE_NEW_ID

We recently were .... confused while looking at IdConflictPolicy.CREATE_NEW_ID (https://jackrabbit.apache.org/filevault/referenceablenodes.html#Id_Conflict_Policies and https://jackrabbit.apache.org/filevault/apidocs/org/apache/jackrabbit/vault/fs/api/IdConflictPolicy.html).

Trouble is, this Enum suggests that it's only relevant when there indeed is an ID conflict. However, it is mapped to https://developer.adobe.com/experience-manager/reference-materials/spec/javax.jcr/javadocs/jcr-2.0/javax/jcr/ImportUUIDBehavior.html which also affects nodes where there actually is no ID collision whatsoever.

I believe it would be hard to change the behaviour at this point, so we should at least improve the documentation.