[glibmm] ActionMap: Improve add_action_with_parameter docs

[Daniel Boles <dboles src gnome org>]

commit ec39b325551ddf59c45441f9c3d848f816d95144
Author: Daniel Boles <dboles src gnome org>
Date:   Fri May 5 10:08:58 2017 +0100

    ActionMap: Improve add_action_with_parameter docs
    
    Elaborate on the purpose of the parameter_type in the new overload.

 gio/src/actionmap.hg |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/gio/src/actionmap.hg b/gio/src/actionmap.hg
index 28b6492..871be0e 100644
--- a/gio/src/actionmap.hg
+++ b/gio/src/actionmap.hg
@@ -92,8 +92,9 @@ public:
    */
   using ActivateWithParameterSlot = sigc::slot<void(const Glib::VariantBase&)>;
 
-  /** A convenience method for creating a SimpleAction instance
-   * and adding it to the ActionMap.
+  /** A convenience method for creating a SimpleAction instance, which when
+   * activated will call a slot receiving a given type of parameter, and adding
+   * that SimpleAction to the ActionMap.
    *
    * @param name The name of the Action.
    * @param parameter_type The type of parameter to be passed to the slot.