chore: comment on slot optimization hint [ci skip]

Author: yyx990803

packages/runtime-core/src/componentSlots.ts

@@ -36,6 +36,10 @@ export type RawSlots = {
   // normalizeChildren when the component vnode is created.
   _ctx?: ComponentInternalInstance | null
   // internal, indicates compiler generated slots
+  // we use a reserved property instead of a vnode patchFlag because the slots
+  // object may be directly passed down to a child component in a manual
+  // render funciton, and the optimization hint need to be on the slot object
+  // itself to be preserved.
   _?: 1
 }