Add note about loosing data for performance/resource overview after restore

[warg]

Hello,

I just noticed that the system view with the overview of the CPU, Memory & Disk usage is lost after restoring from a full backup. This isn't too bad and could be expected behavior but it's not mentioned in the docs at https://docs.cloudron.io/backups/. In my case it's no big deal although I would need the old data to answer a question about a bottleneck I face in the forum so now I lost that data to answer it.

Best Regards,

[girish]

Right, the historic graphs data is not retained since it's all very server specific. And most likely won't even map properly to the new server. I can put a note in the docs.