msglist [nfc]: Comment more about another inset, for completeness

lib/widgets/compose_box.dart

@@ -1278,6 +1278,7 @@ class _ErrorBanner extends StatelessWidget {
 /// The compose box.
 ///
 /// Takes the full screen width, covering the horizontal insets with its surface.
+/// Also covers the bottom inset with its surface.
 class ComposeBox extends StatefulWidget {
   ComposeBox({super.key, required this.narrow})
     : assert(ComposeBox.hasComposeBox(narrow));

lib/widgets/message_list.dart

@@ -298,6 +298,7 @@ class _MessageListPageState extends State<MessageListPage> implements MessageLis
         builder: (BuildContext context) => Column(
           // Children are expected to take the full horizontal space
           // and handle the horizontal device insets.
+          // The bottom inset should be handled by the last child only.
           children: [
             MediaQuery.removePadding(
               // Scaffold knows about the app bar, and so has run this
@@ -459,6 +460,8 @@ const kFetchMessagesBufferPixels = (kMessageListFetchBatchSize / 2) * _kShortMes
 ///
 /// Takes the full screen width, keeping its contents
 /// out of the horizontal insets with transparent [SafeArea] padding.
+/// When there is no [ComposeBox], also takes responsibility
+/// for dealing with the bottom inset.
 class MessageList extends StatefulWidget {
   const MessageList({super.key, required this.narrow, required this.onNarrowChanged});
 
@@ -572,7 +575,9 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
                   bottom: 0,
                   right: 0,
                   // TODO(#311) SafeArea shouldn't be needed if we have a
-                  //   bottom nav. That will pad the bottom inset.
+                  //   bottom nav; that will pad the bottom inset. Remove it,
+                  //   and the mention of bottom-inset handling in
+                  //   MessageList's dartdoc.
                   child: SafeArea(
                     child: ScrollToBottomButton(
                       scrollController: scrollController,
@@ -623,8 +628,8 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
         }));
 
     if (!ComposeBox.hasComposeBox(widget.narrow)) {
-      // TODO(#311) If we have a bottom nav, it will pad the bottom
-      //   inset, and this shouldn't be necessary
+      // TODO(#311) If we have a bottom nav, it will pad the bottom inset,
+      //   and this can be removed; also remove mention in MessageList dartdoc
       sliver = SliverSafeArea(sliver: sliver);
     }