Hi,
Security Rules are used to "Block Packets".
The option "Enable port scan detection" is a method to identify a pattern in blocked packets and compare it to the port scan pattern.
As specified in the description provided in the policy: "A security rule needs to be created to block traffic".
The description in the help file is talking about what is a port scan. Where as the one in the policy is talking about how Symantec detects a pattern in it.
Aniket