add some more info.

content/en/hub/tasks/advanced/tunables.md

@@ -34,4 +34,13 @@ the value to use for the
 or [rc.conf](https://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/config-tuning.html).
 An optional description can be given. If you wish to create the system tunable
 but not immediately enable it, unset the *Enable* checkbox. Configured tunables
-remain in effect until deleted or *Enabled* is unset.
+remain in effect until deleted or *Enabled* is unset.
+
+Restarting the TrueNAS system after making sysctl changes is recommended.
+Some sysctls only take effect at system startup, and restarting the system
+guarantees that the setting values correspond with what is being used by the
+running system.
+
+Be careful when adding or editing the default tunables. Changing the default
+tunables can make the system unusable.
+