document protocol extensions and their interface stability

There are user requests to be able to eject items from memory programmatically from client libraries. Currently it's very unclear what in command_ids.h is intended as public interface and what is not.

We have implemented this in some changes, but we're going to hold it until the interface is clear. Need guidance (and ideally full documentation) on this from the ep_engine team.

Careful attention should be paid to this decision because once it's public interface, any operations need to be properly supported long term.