Tweak docs for [SKNodeDescriptor -identifierForInvalidation:]

Summary: It took me a few tries to understand what this was for.

Reviewed By: jknoxville

Differential Revision: D20462208

fbshipit-source-id: f6dac7836409c8d4ec1ffb90a74f632624fb68f7

iOS/Plugins/FlipperKitLayoutPlugin/FlipperKitLayoutPlugin/SKNodeDescriptor.h

@@ -45,9 +45,13 @@ typedef void (^SKNodeUpdateData)(id value);
 - (NSString*)identifierForNode:(T)node;
 
 /**
-  An ID which is equal between reflowing components is needed to get the
-  identifier of root node of a tree which need to be invalidated on
-  FlipperKitLayoutPlugin side.
+  When the setData command is received from Flipper to change a node's data,
+  an "invalidateWithData" command is sent back to signal that the node has
+  changed. However sometimes you may want to invalidate some other node,
+  not the node that had its data actually modified; usually some ancestor.
+  This method allows you to substitute another node's identifier.
+  If you do not override it, the default behavior is to simply return
+  the node's identifier.
 */
 - (NSString*)identifierForInvalidation:(T)node;