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>

src/google/protobuf/go_features.proto

diff --git a/src/google/protobuf/go_features.proto b/src/google/protobuf/go_features.proto
index a7e14f8..a9cc792 100644
--- a/src/google/protobuf/go_features.proto
+++ b/src/google/protobuf/go_features.proto
@@ -19,6 +19,7 @@
 
 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,

types/gofeaturespb/go_features.pb.go

diff --git a/types/gofeaturespb/go_features.pb.go b/types/gofeaturespb/go_features.pb.go
index 61e3f76..e0b72ea 100644
--- a/types/gofeaturespb/go_features.pb.go
+++ b/types/gofeaturespb/go_features.pb.go
@@ -148,6 +148,7 @@
 type GoFeatures struct {
 	state protoimpl.MessageState `protogen:"open.v1"`
 	// Whether or not to generate the deprecated UnmarshalJSON method for enums.
+	// Can only be true for proto using the Open Struct api.
 	LegacyUnmarshalJsonEnum *bool `protobuf:"varint,1,opt,name=legacy_unmarshal_json_enum,json=legacyUnmarshalJsonEnum" json:"legacy_unmarshal_json_enum,omitempty"`
 	// One of OPEN, HYBRID or OPAQUE.
 	ApiLevel        *GoFeatures_APILevel        `protobuf:"varint,2,opt,name=api_level,json=apiLevel,enum=pb.GoFeatures_APILevel" json:"api_level,omitempty"`