Fake user warning for new users

The concept of "fake user" makes sense when you know about it, but it's not obvious to new users, and you can lose your work if you don't know how it works. When I started using Blender I lost a lot of work because I didn't realise my textures would be deleted, and it was super frustrating. A quick web search shows I'm by far not the only person to have been bitten by it.

There have been lots of proposals to change the fake user system itself, but instead I want to propose a very minimal change that would resolve the discoverability issue without affecting existing workflows.

The idea is just to have a warning popup the first time you save a file with unused data blocks. It would look something like this:

Warning: the following data blocks are not used and will not be saved. To include them in the save file, set them to 'fake user' or make sure they are used in a scene. See [link to manual] for more details.

 [list of unused data blocks]

◻️ do not show this again

This warning would only appear on new installs, and it would easy to disable. So it would have basically no effect for those who don't need it, but it would let beginners know about an unusual feature that might otherwise delete their work unexpectedly, and it could be useful for some experienced users as well.