storage: mark legacy storage classes

Mark:
* DURABLE_REDUCED_AVAILABILITY
* MULTI_REGIONAL
* REGIONAL
as legacy storage classes.

Fixes #1501

Change-Id: I74b27565f5a8d9fb5da7b81a6a968dfdf38e724f
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/42730
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jean de Klerk <deklerk@google.com>
diff --git a/storage/bucket.go b/storage/bucket.go
index d585e3e..07c470d 100644
--- a/storage/bucket.go
+++ b/storage/bucket.go
@@ -272,6 +272,9 @@
 	// "DURABLE_REDUCED_AVAILABILITY". Defaults to "STANDARD", which
 	// is equivalent to "MULTI_REGIONAL" or "REGIONAL" depending on
 	// the bucket's location settings.
+	//
+	// "DURABLE_REDUCED_AVAILABILITY", "MULTI_REGIONAL" and "REGIONAL"
+	// are considered legacy storage classes.
 	StorageClass string
 
 	// Created is the creation time of the bucket.