Document the workings of the plugin cache, and how to flush it

Plugins not loading is often because during a previous run the environment was not setup correctly, and then Qt would have tagged the plugin as broken in the cache. After fixing the environment the plugin will fail to load because of the cache, even though everything is good.

Either fix the cache mechanism to not store a plugin as broken if the result indicates that the loading failed due to a missing dependency; or document how the cache works and how to clean it for each platform.