[gtk] Overlay: Document overlay children aren't measured

[Daniel Boles <dboles src gnome org>]

commit a7a0a34da101bab9f08dcb68968d65fbae6d362d
Author: Daniel Boles <dboles src gmail com>
Date:   Thu Jun 13 18:22:33 2019 +0100

    Overlay: Document overlay children aren't measured
    
    Some users expect that the Overlay will automatically request enough
    size for its overlay children as well as its main child. It doesn't,
    because it's just a GtkBin. Add a short paragraph pointing that out.
    
    Close https://gitlab.gnome.org/GNOME/gtk/issues/1939

 gtk/gtkoverlay.c | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/gtk/gtkoverlay.c b/gtk/gtkoverlay.c
index 7001aa59ba..f23982221e 100644
--- a/gtk/gtkoverlay.c
+++ b/gtk/gtkoverlay.c
@@ -50,6 +50,9 @@
  * More complicated placement of overlays is possible by connecting
  * to the #GtkOverlay::get-child-position signal.
  *
+ * An overlay’s minimum and natural sizes are those of its main child. The sizes
+ * of overlay children are not considered when measuring these preferred sizes.
+ *
  * # GtkOverlay as GtkBuildable
  *
  * The GtkOverlay implementation of the GtkBuildable interface