[gtkmm] Container: Add an intro line to match gtkmm-3-24

[Daniel Boles <dboles src gnome org>]

commit 57c14d98fbdb971d4dad5598e24690be6a91b480
Author: Daniel Boles <dboles src gmail com>
Date:   Sat Oct 20 21:30:12 2018 +0100

    Container: Add an intro line to match gtkmm-3-24
    
    This is just to bring master up to date with my simultaneous commit to 3

 gtk/src/container.hg | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/gtk/src/container.hg b/gtk/src/container.hg
index 40e9d282..15f0014c 100644
--- a/gtk/src/container.hg
+++ b/gtk/src/container.hg
@@ -76,7 +76,8 @@ public:
   /// Request that contained widgets check their size
   _WRAP_METHOD(void check_resize(), gtk_container_check_resize)
 
-  /** For instance,<br>
+  /** A slot to invoke with each child iterated over by foreach() or forall().
+   * For instance,<br>
    * void on_foreach(Gtk::Widget& widget);
    */
   using ForeachSlot = sigc::slot<void(Widget&)>;