[devhelp] docs: add "Since: 3.26" to dh_book_manager_get_singleton()

[Sébastien Wilmet <swilmet src gnome org>]

commit f141ac66176d9c208e9fa3acd9dbdeeda0fb650f
Author: Sébastien Wilmet <sebastien wilmet uclouvain be>
Date:   Wed May 3 19:54:00 2017 +0200

    docs: add "Since: 3.26" to dh_book_manager_get_singleton()

 src/dh-book-manager.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/src/dh-book-manager.c b/src/dh-book-manager.c
index 6188458..47a25f7 100644
--- a/src/dh-book-manager.c
+++ b/src/dh-book-manager.c
@@ -821,6 +821,7 @@ dh_book_manager_new (void)
  * dh_book_manager_get_singleton:
  *
  * Returns: (transfer none): the #DhBookManager singleton instance.
+ * Since: 3.26
  */
 DhBookManager *
 dh_book_manager_get_singleton (void)