DOC: clarify that check-sni needs an argument.

Make it more obvious that check-sni requires an argument, and that
it can only be a string. Also refer to sni for proxied traffic.

This can be backported to 1.8 as check-sni appeared in 1.8.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 071f0c5..e6e5481 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -11563,9 +11563,10 @@
   "check-send-proxy" option needs to be used to force the use of the
   protocol. See also the "send-proxy" option for more information.
 
-check-sni
+check-sni <sni>
   This option allows you to specify the SNI to be used when doing health checks
-  over SSL.
+  over SSL. It is only possible to use a string to set <sni>. If you want to
+  set a SNI for proxied traffic, see "sni".
 
 check-ssl
   This option forces encryption of all health checks over SSL, regardless of