docs(storage): Update Bucket#default_kms_key= docs

* Demonstrate deleting the Cloud KMS encryption key

pr: #7845
refs: #7823

google-cloud-storage/lib/google/cloud/storage/bucket.rb

@@ -557,7 +557,8 @@ def default_kms_key
         # Set the Cloud KMS encryption key that will be used to protect files.
         # For example: `projects/a/locations/b/keyRings/c/cryptoKeys/d`
         #
-        # @param [String] new_default_kms_key New Cloud KMS key name.
+        # @param [String, nil] new_default_kms_key New Cloud KMS key name, or
+        #   `nil` to delete the Cloud KMS encryption key.
         #
         # @example
         #   require "google/cloud/storage"
@@ -571,6 +572,15 @@ def default_kms_key
         #
         #   bucket.default_kms_key = kms_key_name
         #
+        # @example Delete the default Cloud KMS encryption key:
+        #   require "google/cloud/storage"
+        #
+        #   storage = Google::Cloud::Storage.new
+        #
+        #   bucket = storage.bucket "my-bucket"
+        #
+        #   bucket.default_kms_key = nil
+        #
         def default_kms_key= new_default_kms_key
           @gapi.encryption = API::Bucket::Encryption.new \
             default_kms_key_name: new_default_kms_key