chore(storage): minor corrections for performance while pushing back unreachable buckets to vector (#15803)

Author: bajajneha27

google/cloud/storage/internal/bucket_requests.cc

@@ -243,10 +243,10 @@ StatusOr<ListBucketsResponse> ListBucketsResponse::FromHttpResponse(
     result.items.emplace_back(std::move(*parsed));
   }
 
-  if (json.count("unreachable") != 0) {
-    for (auto const& kv : json["unreachable"].items()) {
-      result.unreachable.emplace_back(kv.value().get<std::string>());
-    }
+  auto const& unreachable = json["unreachable"];
+  result.unreachable.reserve(unreachable.size());
+  for (auto const& bucket : unreachable) {
+    result.unreachable.push_back(bucket.get<std::string>());
   }
 
   return result;

google/cloud/storage/well_known_parameters.h

@@ -595,7 +595,7 @@ struct RequestedPolicyVersion
  *
  * By default requests listing resources only includs the latest (live) version
  * of each resource, set this option to `true` to get list of unreachable
- * regions.
+ * resources.
  */
 struct ReturnPartialSuccess
     : public internal::WellKnownParameter<ReturnPartialSuccess, bool> {