give a link to Vue 2 slots docs page for users who are coming from Vue <2.6.0

I personally had a hard-time finding exact changes needed to be done and I had to constantly switch tabs between Vue 2 slots page, migration guide to Vue 3, and Vue 3 docs page for more info. I belive leaving a link that informs the migration is a little different for Vue <2.6.0 users is helpful.

Author: aghArdeshir

src/breaking-changes/slots-unification.md

@@ -64,4 +64,8 @@ A majority of the change has already been shipped in 2.6. As a result, the migra
 1. Replace all `this.$scopedSlots` occurrences with `this.$slots` in 3.x.
 2. Replace all occurrences of `this.$slots.mySlot` with `this.$slots.mySlot()`
 
+If you are upgrading to Vue 3 from Vue <2.6.0, you can look at the
+[documentation of slots in Vue 2 docs][https://v2.vuejs.org/v2/guide/components-slots]
+to gain more information on how the slots are changed.
+
 [Migration build flag: `INSTANCE_SCOPED_SLOTS`](../migration-build.html#compat-configuration)