doc: Mention encryption for memcached connections

doc/nghttpx.h2r

@@ -150,6 +150,10 @@ insert serialized session data to memcached with
 as a memcached entry key, with expiry time 12 hours.  Session timeout
 is set to 12 hours.
 
+By default, connections to memcached server are not encrypted.  To
+enable encryption, use :option:`--tls-session-cache-memcached-tls`
+option.
+
 TLS SESSION TICKET RESUMPTION
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -189,6 +193,10 @@ used, LEN must be 48.  If
 keys.  The key appeared first is used as encryption key.  All the
 remaining keys are used as decryption only.
 
+By default, connections to memcached server are not encrypted.  To
+enable encryption, use :option:`--tls-ticket-key-memcached-tls`
+option.
+
 If :option:`--tls-ticket-key-file` is given, encryption key is read
 from the given file.  In this case, nghttpx does not rotate key
 automatically.  To rotate key, one has to restart nghttpx (see