msglist [nfc]: Explain an odd-looking Center widget that we want to keep

Looks like I explained the Center widget in the commit message that
added it -- ef02ee4 -- but I think the comment is helpful. At
first, it looked like another case of a no-op Center widget like in
the previous commit, but it turns out it does have a desirable
effect in the horizontal axis.

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -467,6 +467,7 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
       //   will have a `MediaQuery.removePadding` with `removeBottom: true`.
       bottom: false,
 
+      // (center horizontally on wide screens; a no-op in the vertical direction)
       child: Center(
         child: ConstrainedBox(
           constraints: const BoxConstraints(maxWidth: 760),