[gnome-software] trivial: Add missing docstring

[Robert Ancell <rancell src gnome org>]

commit baf8b357e05b97c33121de399179430940e0ae39
Author: Robert Ancell <robert ancell canonical com>
Date:   Thu Oct 15 14:33:51 2015 +0100

    trivial: Add missing docstring

 src/gs-app.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/src/gs-app.c b/src/gs-app.c
index ac48df5..e046b70 100644
--- a/src/gs-app.c
+++ b/src/gs-app.c
@@ -2325,6 +2325,9 @@ gs_app_class_init (GsAppClass *klass)
                                   G_PARAM_READWRITE | G_PARAM_CONSTRUCT);
        g_object_class_install_property (object_class, PROP_PROGRESS, pspec);
 
+       /**
+        * GsApp:install-date:
+        */
        pspec = g_param_spec_uint64 ("install-date", NULL, NULL,
                                     0, G_MAXUINT64, 0,
                                     G_PARAM_READWRITE | G_PARAM_CONSTRUCT);