docs: Add @since for MaybeBool.

diff --git a/folks/types.vala b/folks/types.vala
index 509699b..fe29d4f 100644 (file)
--- 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
     {