Clarify if App Cache is only for Linux

[Piedone]

The https://github.com/Azure-App-Service/KuduLite/wiki/App-Cache docs page heavily hints on App Cache only being a feature of Linux App Services, and the Local Cache docs warns about it being a Linux substitute to the Windows-only Local Cache but it's not explicitly stated that Windows is not supported. From my testing, it seems it's a Linux-only feature (in any case, configuring it doesn't seem to help under Windows).

BTW I'm working on the OrchardCMS/OrchardCore#14859 issue where it seems our performance problems with an Orchard Core feature are due to file IO being slow under a Windows App Service.