Better id doc

shell/platform/embedder/embedder.h

@@ -266,13 +266,10 @@ typedef enum {
 
 typedef struct _FlutterEngine* FLUTTER_API_SYMBOL(FlutterEngine);
 
-/// Identifier for views.
+/// Unique identifier for views.
 ///
-/// How view IDs are generated is decided by the embedding. The engine does not
-/// assume any specifics about the generation algorithm, including whether it
-/// utilizes the entire range of int64_t or permits the use of zero or negative
-/// IDs. The only requirement is that at any given moment, no two views share
-/// the same ID.
+/// View IDs are generated by the embedder and are
+/// opaque to the engine; the engine does not interpret view IDs in any way.
 typedef int64_t FlutterViewId;
 
 typedef struct {