Document forward-compatibility guarantees for RPC protocol

To get the RPC protocol working, we should document in rpc-meta-draft.md what our guarantees are for forward compatibility. This boils down to ensuring that if a client is using a method or a feature that Arti doesn't know about, the client has some confidence of being able to implement reasonable fallback behavior.

From our notes:

proposal: for now our story can be "we do breaking changes that give you an error, but not breaking changes that change behavior."

later: if we get "method unknown" error, then try fallback.

later: expose all schema.

Must promise: No critical flags.

Never set reject_unknown_fields.

Provide an option to require a feature before running a method.

I should expand those into a spec patch before I forget what they mean.