Document that we intercept WriteHeader already

internal/git/git-http.go

@@ -40,7 +40,8 @@ func postRPCHandler(a *api.API, name string, handler func(*GitHttpResponseWriter
 
 		if err := handler(w, r, ar); err != nil {
 			// If the handler already wrote a response this WriteHeader call is a
-			// (harmless) no-op.
+			// no-op. It never reaches net/http because GitHttpResponseWriter calls
+			// WriteHeader on its underlying ResponseWriter at most once.
 			w.WriteHeader(500)
 			helper.LogError(r, fmt.Errorf("%s: %v", name, err))
 		}