Update config.sample.php

Comments to clarify the purpose of session_keepalive.
Fixes issue in Nextcloud Documentation (nextcloud/documentation#7244)

Signed-off-by: HelloDarkness <111370255+hellodarkness@users.noreply.github.com>

Author: hellodarkness

config/config.sample.php

@@ -339,8 +339,10 @@
 
 /**
  * Enable or disable the automatic logout after session_lifetime, even if session
- * keepalive is enabled. This will make sure that an inactive browser will be logged out
- * even if requests to the server might extend the session lifetime.
+ * keepalive is enabled. This will make sure that an inactive browser will log itself out
+ * even if requests to the server might extend the session lifetime. Note: the logout is   
+ * handled on the client side. This is not a way to limit the duration of potentially
+ * compromized sessions.
  *
  * Defaults to ``false``
  */