Clarity around the sender/receiver in ShouldDeliverCallback. (#1050)

## Changes

<!-- Please summarize your changes. -->

Elaborated on the sender/receiver structure of
`TextChannel.ShouldDeliverCallback`. My co-worker got confused while he
was working on a problem with us and thought the callback only gave you
information about who sent the message, since in Play Solo you would be
the only sender and recipient of the message. After confirming how I
suspected it actually worked, I went in and made this documentation
change so others hopefully don't have similar confusion in the future.

<!-- Please link to any applicable information (forum posts, bug
reports, etc.). -->

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---------

Author: MaximumADHD

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

@@ -293,17 +293,23 @@ callbacks:
       returns anything else (including `nil`), the message won't be delivered to
       that client, although the sender will see the message regardless.
 
-      The sender can be referenced by `Class.TextChatMessage.TextSource`.
+      The sender can be referenced by `Class.TextChatMessage.TextSource`,
+      while the receiver is the `textSource` argument. Note that the sender
+      and receiver can be the same, as the callback iterates through all possible
+      receivers. In Roblox Studio, you will be the only sender and recipient of a
+      message while in solo **Play** mode.
     code_samples: []
     parameters:
       - name: message
         type: TextChatMessage
         default:
-        summary: ''
+        summary: |
+          The message being sent, which also contains the sender of the message.
       - name: textSource
         type: TextSource
         default:
-        summary: ''
+        summary: |
+          The `Class.TextSource` of the user who will be receiving the message.
     returns:
       - type: Tuple
         summary: ''