[DOCS] Update passphrase security setting (elastic/x-pack-elasticsearch#2431)
Original commit: elastic/x-pack-elasticsearch@f9accd222c2c91a9e57f60f56bc1547f9d4e7726
@@ -79,7 +79,9 @@ Set to `false` to disable the built-in token service. Defaults to `true` unless
`xpack.security.authc.token.passphrase`::
A secure passphrase that must be the same on each node and greater than
8 characters in length. This passphrase is used to derive a cryptographic key
-with which the tokens will be encrypted and authenticated.
+with which the tokens will be encrypted and authenticated. If this setting is
+not used, the cluster automatically generates a key, which is the recommended
+method.
`xpack.security.authc.token.timeout`::
The length of time that a token is valid for. By default this value is `20m` or
