Warn about client-side tags being dropped (#967)

## Changes

This behavior can cause unexpected removal of tags added locally.

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---------

Author: Ketasaja

content/en-us/reference/engine/classes/CollectionService.yaml

@@ -51,10 +51,16 @@ methods:
       signal created by `Class.CollectionService:GetInstanceAddedSignal()` with
       the given tag.
 
-      **Warning:** When tagging an instance, it is common that some resources
-      are used to give the tag its functionality, e.g. event connections or
-      tables. To prevent memory leaks, it is a good idea to clean these up
-      (disconnect, set to nil, etc) when no longer needed for a tag. Do this
+      ##### Warnings
+      
+      - An instance's tags that were added client-side will be dropped if the server
+      later adds or removes a tag on that instance because the server replicates all
+      tags together and overwrites previous tags.
+
+      - When tagging an instance, it is common that some resources
+      are used to give the tag its functionality, for example event connections or
+      tables. To prevent memory leaks, it's a good idea to clean these up
+      (disconnect, set to `nil`, etc.) when no longer needed for a tag. Do this
       when calling `Class.CollectionService:RemoveTag()`, calling
       `Class.Instance:Destroy()` or in a function connected to a signal returned
       by `Class.CollectionService:GetInstanceRemovedSignal()`.

content/en-us/reference/engine/classes/Instance.yaml

@@ -321,7 +321,13 @@ methods:
       already applied. Successfully adding a tag will fire a signal created by
       `Class.CollectionService:GetInstanceAddedSignal()` with the given tag.
 
-      Note that when tagging an instance, it's common that some resources are
+      ##### Warnings
+      
+      - An instance's tags that were added client-side will be dropped if the server
+      later adds or removes a tag on that instance because the server replicates
+      all tags together and overwrites previous tags.
+
+      - When tagging an instance, it is common that some resources are
       used to give the tag its functionality, for example event connections or
       tables. To prevent memory leaks, it's a good idea to clean these up
       (disconnect, set to `nil`, etc.) when no longer needed for a tag. Do this