The setConfigIOProxy function should clear the caches

[doug-walker]

Creating this issue to capture the follow-up required on PR #2128.

1. It would make sense to have setConfigIOProxy clear the file and processor caches. This is necessary and a developer may forget to do this manually.
2. It would be helpful to add some additional info to OpenColorIO.h to explain the difference between CreateFromConfigIOProxy and setConfigIOProxy. (The current info on setConfigIOProxy is a bit misleading since it says it could provision the config.) And the Context one could be updated to refer to the Config one. Please refer to the info Cedrik provided on Slack, which was copied into PR #2128.