Update StarterGui.yaml (#1080)

Add warning for MakeChatSystemMessage regarding TextChatService
incompatibility; Using MakeChatSystemMessage on modern TextChatService
does not work

## Changes

I added a warning to `MakeChatSystemMessage` method about
LegacyChatService requirement.
This clarifies that this method only works with the deprecated
LegacyChatService and points users to the TextChatService alternative
'DisplaySystemMessage'.

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---------

Co-authored-by: IgnisRBX <[email protected]>

Author: samerop

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

@@ -340,8 +340,10 @@ methods:
       </table>
 
       ##### ChatMakeSystemMessage
-
-      Display a formatted message in the chat.
+      Display a formatted message in the chat. Using this method requires the experience's
+      `Class.TextChatService.ChatVersion` to be set to `Enum.ChatVersion|LegacyChatService`, although
+      legacy chat is **deprecated** and usage is discouraged. For experiences using the current
+      `Class.TextChatService`, refer to `Class.TextChannel:DisplaySystemMessage()`.
 
       <table size="small">
       	<thead>