[gnome-builder] flatpak: add docs to state-dir property

[Christian Hergert <chergert src gnome org>]

commit 95406a989a1238f5c4fa97cb4be36635a11674b6
Author: Christian Hergert <chergert redhat com>
Date:   Wed Jan 10 13:26:06 2018 -0800

    flatpak: add docs to state-dir property

 src/plugins/flatpak/gbp-flatpak-download-stage.c |   13 ++++++++++++-
 1 files changed, 12 insertions(+), 1 deletions(-)
---
diff --git a/src/plugins/flatpak/gbp-flatpak-download-stage.c 
b/src/plugins/flatpak/gbp-flatpak-download-stage.c
index 430321d..bb60fc9 100644
--- a/src/plugins/flatpak/gbp-flatpak-download-stage.c
+++ b/src/plugins/flatpak/gbp-flatpak-download-stage.c
@@ -161,8 +161,19 @@ gbp_flatpak_download_stage_class_init (GbpFlatpakDownloadStageClass *klass)
 
   stage_class->query = gbp_flatpak_download_stage_query;
 
+  /**
+   * GbpFlatpakDownloadStage:state-dir:
+   *
+   * The "state-dir" is the flatpak-builder state directory, to be used
+   * as a parameter to "flatpak-builder --state-dir".
+   *
+   * Since: 3.28
+   */
   properties [PROP_STATE_DIR] =
-    g_param_spec_string ("state-dir", NULL, NULL, NULL,
+    g_param_spec_string ("state-dir",
+                         "State Dir",
+                         "The flatpak-builder state directory",
+                         NULL,
                          G_PARAM_WRITABLE | G_PARAM_CONSTRUCT_ONLY | G_PARAM_STATIC_STRINGS);
 
   g_object_class_install_properties (object_class, N_PROPS, properties);