updater server-docs needs improvement

[santanair]

https://doc.owncloud.com/branded_clients/next/branded_desktop_client/update_branded_desktop_clients.html

• The yaml provided as an example seems out of date
• win32 platform must not be updated
• there is an alias-platforms section but that is not mentioned in the doc
• meaning of /pubDate/ field needs to be clarified
• win64 and AppImage platforms are not mentioned in the doc
• For platform Linux the /currentVersion/ is the "Exact version of the new client, including the build number" but in the yaml build number is missing
• The debugging for Linux seems not to be documented
• explain if macOS macOS branded clients support the virtual file system or not. (option is present in the original (non branded) version 3 client.)

[mmattel]

@michaelstingl can you or someone from your team provide a PR that fixes the required changes?
I have no knowledge about the changes necessary.