[devhelp] Sidebar: better document Private struct

[Sébastien Wilmet <swilmet src gnome org>]

commit e36a16cd336d2bed8ac930de643af3cc441c2820
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Thu Jan 4 21:09:26 2018 +0100

    Sidebar: better document Private struct

 src/dh-sidebar.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/src/dh-sidebar.c b/src/dh-sidebar.c
index d2be36b..26da63c 100644
--- a/src/dh-sidebar.c
+++ b/src/dh-sidebar.c
@@ -53,6 +53,7 @@
  */
 
 typedef struct {
+        /* A GtkSearchEntry. */
         GtkEntry *entry;
 
         DhBookTree *book_tree;