[devhelp] help: add app short description

[Sébastien Wilmet <swilmet src gnome org>]

commit d8a4b8d90f3e723e636e3832454afa8c36e3d41d
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Wed Dec 18 18:29:01 2019 +0100

    help: add app short description

 help/C/index.page | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/help/C/index.page b/help/C/index.page
index db1646bc..aaace987 100644
--- a/help/C/index.page
+++ b/help/C/index.page
@@ -2,4 +2,10 @@
       type="guide"
       id="index">
 <title>Devhelp User Documentation</title>
+
+<synopsis>
+  <p>
+    Devhelp is a developer tool for browsing and searching API documentation.
+  </p>
+</synopsis>
 </page>