doc(storage): update gRPC docs (#9488)

Update gRPC docs with instructions on how to allowlist buckets
for the gRPC API.

storage/doc.go

@@ -335,9 +335,10 @@ to add a [custom audit logging] header:
 
 This package includes support for the Cloud Storage gRPC API, which is currently
 in preview. This implementation uses gRPC rather than the current JSON & XML
-APIs to make requests to Cloud Storage. If you would like to try the API,
-please contact your GCP account rep for more information. The gRPC API is not
-yet generally available, so it may be subject to breaking changes.
+APIs to make requests to Cloud Storage. Kindly contact the Google Cloud Storage gRPC
+team at [email protected] with a list of GCS buckets you would like to
+allowlist to access this API. The Go Storage gRPC library is not yet generally
+available, so it may be subject to breaking changes.
 
 To create a client which will use gRPC, use the alternate constructor:
 
@@ -349,7 +350,7 @@ To create a client which will use gRPC, use the alternate constructor:
 	// Use client as usual.
 
 If the application is running within GCP, users may get better performance by
-enabling DirectPath (enabling requests to skip some proxy steps). To enable,
+enabling Google Direct Access (enabling requests to skip some proxy steps). To enable,
 set the environment variable `GOOGLE_CLOUD_ENABLE_DIRECT_PATH_XDS=true` and add
 the following side-effect imports to your application: