Clarify PointerSignalResolver.register (#127605)

packages/flutter/lib/src/gestures/pointer_signal_resolver.dart

@@ -61,8 +61,21 @@ class PointerSignalResolver {
 
   /// Registers interest in handling [event].
   ///
-  /// See the documentation for the [PointerSignalResolver] class on when and
-  /// how this method should be used.
+  /// This method may be called multiple times (typically from different parts of the
+  /// widget hierarchy) for the same `event`, with differenet `callback`s, as the event
+  /// is being dispatched across the tree. Once the dispatching is complete, the
+  /// [GestureBinding] calls [resolve], and the first registered callback is called.
+  ///
+  /// The `callback` is invoked with one argument, the `event`.
+  ///
+  /// Once the [register] method has been called with a particular `event`, it must
+  /// not be called for other `event`s until after [resolve] has been called. Only one
+  /// event disambiguation can be in flight at a time. In normal use this is achieved
+  /// by only registering callbacks for an event as it is actively being dispatched
+  /// (for example, in [Listener.onPointerSignal]).
+  ///
+  /// See the documentation for the [PointerSignalResolver] class for an example of
+  /// using this method.
   void register(PointerSignalEvent event, PointerSignalResolvedCallback callback) {
     assert(_currentEvent == null || _isSameEvent(_currentEvent!, event));
     if (_firstRegisteredCallback != null) {