compose [nfc]: Rename and document a helper

_ComposeBoxContainer also supports an error banner that doesn't
replace the text inputs. We don't use that feature yet (the Figma
has some examples where it'll be helpful:
  https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=4031-16992&m=dev
and we used it in a draft toward zulip#720 that wasn't merged:
  zulip#720 (comment)
) but it seems helpful anyway to nail down this helper like this.

Author: chrisbobbe

lib/widgets/compose_box.dart

@@ -1521,7 +1521,8 @@ class _ComposeBoxState extends State<ComposeBox> with PerAccountStoreAwareStateM
     super.dispose();
   }
 
-  Widget? _errorBanner(BuildContext context) {
+  /// An [_ErrorBanner] that replaces the compose box's text inputs.
+  Widget? _errorBannerComposingNotAllowed(BuildContext context) {
     final store = PerAccountStoreWidget.of(context);
     switch (widget.narrow) {
       case ChannelNarrow(:final streamId):
@@ -1553,7 +1554,7 @@ class _ComposeBoxState extends State<ComposeBox> with PerAccountStoreAwareStateM
   Widget build(BuildContext context) {
     final Widget? body;
 
-    final errorBanner = _errorBanner(context);
+    final errorBanner = _errorBannerComposingNotAllowed(context);
     if (errorBanner != null) {
       return _ComposeBoxContainer(body: null, errorBanner: errorBanner);
     }