docs(storage): add GCS+gRPC release notes (#11760)

Author: coryan

google/cloud/storage/doc/storage-grpc.dox

@@ -5,8 +5,40 @@
 The GCS C++ client library includes an optional plugin to access GCS via gRPC.
 When using GCS from Google Compute Engine (GCE) this plugin can enable higher
 total throughput across large workloads that run on hundreds or thousands of
-VMs.  Note that this feature is not GA, consult with with account manager to
-be included in the early access.
+VMs.  Note that this feature is not GA. Consult with your account manager to be
+included in the early access program.
+
+## Release Notes
+
+There are some features that the gRPC plugin does not implement or implements
+differently from the REST-based implementation.
+
+1. For best performance consider using Protobuf >= 23.2 and gRPC >= 1.55.0.
+   Older versions of Protobuf requires enabling [ctype=CORD workarounds] at
+   compile-time.
+1. To override the default endpoint you should use
+   [`EndpointOption`](@ref google::cloud::EndpointOption) instead of
+   [`RestEndpointOption`](@ ref google::cloud::storage::RestEndpointOption).
+1. The default endpoint (`storage.googleapis.com`) works from any hosting
+   environment (on-prem, GKE, GCE, other cloud providers, etc.). For best
+   performance on GCE or GKE consider using
+   `google-c2p:///storage.googleapis.com`.
+1. The [`Fields`](@ref google::cloud::storage::Fields) request parameter is
+   passed verbatim to the backend. Some fields have different names in gRPC, and
+   the backend does not translate them. This is a rarely used feature, and we do
+   not anticipate the behavior change is likely to cause problems. If it does
+   cause any issues please [file a bug].
+1. All the `storage::Client::*AccessControl()` functions are emulated. For
+   functions that change state, the emulation uses an [OCC loop]. While this
+   preserves the semantics of a single RPC (e.g. no other concurrent changes
+   are lost), it requires making at least two RPCs. This may increase the cost
+   of calling these functions. Our telemetry shows these functions are not used
+   via the C++ client library, but you should keep this in mind before deciding
+   to use these functions in new code.
+
+[OCC loop]: https://en.wikipedia.org/wiki/Optimistic_concurrency_control
+[file a bug]: https://github.com/googleapis/google-cloud-cpp/issues/new/choose
+[ctype=CORD workarounds]: https://github.com/googleapis/google-cloud-cpp/blob/main/doc/ctype-cord-workarounds.md
 
 ## Required code changes