add documentation for redirect parameters

roles/redirects/defaults/main.yml

+# Every entry creates a redirect listening on port 80 and 443 with the following parameters:
+#   - domain: the domain to listen on
+#   - to: the redirect target as defined by the nginx return statement
+#   - type: HTTP status code to use (302 = temporary redirect, 301 = permanent redirect
+#)
 redirects:
   - static.conf:
     domain: static.conf.archlinux.org