Add upgrade warning to extensions field

log.go

@@ -52,6 +52,15 @@ type Log struct {
 	// is up to the client of the library to properly modify this as it adds
 	// layers and remove those layers when appropriate. This value is a part of
 	// the log, so very large values could cause timing issues.
+	//
+	// N.B. It is _up to the client_ to handle upgrade paths. For instance if
+	// using this with go-raftchunking, the client should ensure that all Raft
+	// peers are using a version that can handle that extension before ever
+	// actually triggering chunking behavior. It is sometimes sufficient to
+	// ensure that non-leaders are upgraded first, then the current leader is
+	// upgraded, but a leader changeover during this process could lead to
+	// trouble, so gating extension behavior via some flag in the client
+	// program is also a good idea.
 	Extensions []byte
 }