DOC: resolve-prefer default value and default-server update

By the code, resolve-prefer defaults to IPv6 and is also available in
the default-server directive.
These information were missing or wrong in the documentation.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index c4060ce..8a59638 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -10097,7 +10097,9 @@
   from the family mentioned in the "resolve-prefer" parameter.
   Available families: "ipv4" and "ipv6"
 
-  Default value: ipv4
+  Default value: ipv6
+
+  Supported in default-server: Yes
 
   Example: server s1 app1.domain.com:80 resolvers mydns resolve-prefer ipv6