src/google/protobuf: document UnmarshalJSON / API level behavior

For golang/protobuf#1657

Change-Id: Icdf7254bced1c0987ff2e969fd096d6eef3918f7
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/635139
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Cassondra Foesch <cfoesch@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: stapelberg

src/google/protobuf/go_features.proto

@@ -19,6 +19,7 @@ extend google.protobuf.FeatureSet {
 
 message GoFeatures {
   // Whether or not to generate the deprecated UnmarshalJSON method for enums.
+  // Can only be true for proto using the Open Struct api.
   optional bool legacy_unmarshal_json_enum = 1 [
     retention = RETENTION_RUNTIME,
     targets = TARGET_TYPE_ENUM,