[folks] docs: Add @since for MaybeBool.

[Travis Reitter <treitter src gnome org>]

commit 3dfc0d6c6a2e46f35948ce24bc0a3b60c9fd9ffb
Author: Travis Reitter <travis reitter collabora co uk>
Date:   Wed Oct 26 11:36:53 2011 -0700

    docs: Add @since for MaybeBool.

 folks/types.vala |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/folks/types.vala b/folks/types.vala
index 509699b..fe29d4f 100644
--- a/folks/types.vala
+++ b/folks/types.vala
@@ -29,6 +29,8 @@ namespace Folks
 {
   /**
    * A 'boolean' type that has a distinct 'unset' state.
+   *
+   * @since 0.3.1
    */
   public enum MaybeBool
     {