MINOR: checks: Add the alpn option for tcp-check connect rules

This option defines which protocols to advertise with ALPN on the SSL conection
opened by a tcp-check connect rule.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 649ded2..327399d 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -9830,6 +9830,11 @@
 
     sni <sni>    specifies the SNI to use to do health checks over SSL.
 
+    alpn <alpn>  defines which protocols to advertise with ALPN. The protocol
+                 list consists in a comma-delimited list of protocol names,
+                 for instance: "http/1.1,http/1.0" (without quotes).
+                 If it is not set, the server ALPN is used.
+
     linger    cleanly close the connection instead of using a single RST.
 
     Examples: