Make existing protocols explicit about proto2

pubsub/README.md

@@ -90,6 +90,7 @@ messages between participating peers.
 The `RPC` protobuf is as follows:
 
 ```protobuf
+syntax = "proto2";
 message RPC {
 	repeated SubOpts subscriptions = 1;
 	repeated Message publish = 2;
@@ -112,6 +113,7 @@ false signifies 'unsubscribe'.
 The RPC message can contain zero or more messages of type 'Message'. The Message protobuf looks like this:
 
 ```protobuf
+syntax = "proto2";
 message Message {
 	optional string from = 1;
 	optional bytes data = 2;
@@ -341,6 +343,7 @@ and may be removed once used.
 The `TopicDescriptor` protobuf is as follows:
 
 ```protobuf
+syntax = "proto2";
 message TopicDescriptor {
 	optional string name = 1;
 	// AuthOpts and EncOpts are unused as of Oct 2018, but

pubsub/gossipsub/gossipsub-v1.0-old.md

@@ -375,6 +375,8 @@ control messages. The four control messages are `ControlIHave` for `IHAVE` messa
 The protobuf is as follows:
 
 ```protobuf
+syntax = "proto2";
+
 message RPC {
     // ...
 	optional ControlMessage control = 3;

pubsub/gossipsub/gossipsub-v1.0.md

@@ -506,6 +506,8 @@ The four control messages are `ControlIHave` for [`IHAVE`](#ihave) messages,
 The protobuf is as follows:
 
 ```protobuf
+syntax = "proto2";
+
 message RPC {
     // ... see definition in pubsub interface spec
 	optional ControlMessage control = 3;

pubsub/gossipsub/gossipsub-v1.1.md

@@ -129,6 +129,7 @@ In this case, the pruned peer will have to rely on the ambient peer discovery se
 The `ControlPrune` message is extended with a `peers` field as follows.
 
 ```protobuf
+syntax = "proto2";
 message ControlPrune {
 	optional string topicID = 1;
 	repeated PeerInfo peers = 2; // gossipsub v1.1 PX