Add explanation about forgetFunc in cacher

Author: wojtek-t

staging/src/k8s.io/apiserver/pkg/storage/cacher/cacher.go

@@ -365,6 +365,8 @@ func (c *Cacher) Watch(ctx context.Context, key string, resourceVersion string,
 
 	// Create a watcher here to reduce memory allocations under lock,
 	// given that memory allocation may trigger GC and block the thread.
+	// Also note that emptyFunc is a placeholder, until we will be able
+	// to compute watcher.forget function (which has to happen under lock).
 	watcher := newCacheWatcher(chanSize, filterWithAttrsFunction(key, pred), emptyFunc, c.versioner)
 
 	// We explicitly use thread unsafe version and do locking ourself to ensure that