Define scope of the google AIPs

[toumorokoshi]

We don't explicitly define the scope of the AIPs today. There should be a section that clarifies what categorize of APIs are not in the AIP scope (e.g. third party protocols that APIs must adhere to, making it impossible to adhere to the AIPs).

There's already a WIP PR on that here: #1130. So this Issue serves as a place for discussion for follow-ups.

e.g. the following section was removed for AIP-111 on planes, since it falls more into a discussion of first vs third party:

### Third Party Management Planes


In the context of the Google AIPs, the management plan in question referes
A method can be considered a management or data operation depending on the
context.

For example, consider a cloud provider which exposes a Kubernetes cluster
resource, which in turn exposes it's own API with resources. In the context of
the cloud, the operations to provision and configure the cluster are *management
plane*, while operations against the Kubernetes cluster are *data plane*.

In the context of the Kubernetes cluster, the operations against the API server
are *management plane*.

The following are common contexts:

- When a first-party service is hosting third-party services, the first-party
  provisioning operations are *management plane* while the operations against
  the third-party service are *data-plane*.

When the plane is discussed in Google AIPs, the context is Google APIs.