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Separate fields for User Documentation, Admin Documentation and Technical Documentation
Create Metadata records for other things that are important to you - eg if you have a lot of documentation artefacts, you may want to create them as specific records, that are then tagged and linked to metadata.
Release Notes: Document things as you normally would, tag it with a Tag for the Release No. then report on the Release Tags as I have done for Features! Voila - Release Notes. You can even add a custom field for Release changes rather than using the User Notes feature.