DOC: usesrc root privileges requirements

The "usesrc" parameter of the source statement requires root privileges.
(cherry picked from commit 91bd337d90cb347feda34b01402f3471c8a4833c)
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 6714afb..64697a4 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -6117,6 +6117,8 @@
   is possible at the server level using the "source" server option. Refer to
   section 5 for more information.
 
+  In order to work, "usesrc" requires root privileges.
+
   Examples :
         backend private
             # Connect to the servers using our 192.168.1.200 source address