Merge pull request #3521 from jsoref/recommended-port

clarify: jnlp agent port help

core/src/main/resources/hudson/security/GlobalSecurityConfiguration/help-slaveAgentPort.html

 <div>
+  If you are not using JNLP agents, it's recommended that you disable this
+  entirely, which is the default installation behavior.
   Jenkins uses a TCP port to communicate with agents launched via JNLP.
-  Normally this port is chosen randomly to avoid collisions, but this would
-  make securing the system difficult. If you are not using JNLP agents,
-  it's recommend to disable this TCP port. Alternatively, you can specify
-  the fixed port number so that you can configure your firewall accordingly.
+  If you're going to use JNLP agents, you can allow the system to randomly
+  select a port at launch (this avoids interfering with other programs,
+  including other Jenkins instances).
+  As it's hard for firewalls to secure a random port, you can instead
+  specify a fixed port number and configure your firewall accordingly.
 </div>