[JENKINS-39161] - Update JNLP4-plaintext documentation according to c…

…omments from @stephenc

CHANGELOG.md

@@ -16,7 +16,7 @@ See (see [Remoting 3 Compatibility Notes](docs/remoting-3-compatibility.md)) for
 Enhancements:
 
 * [JENKINS-36871](https://issues.jenkins-ci.org/browse/JENKINS-36871) - 
-<code>JNLP4-connect</code> and <code>JNLP4-plaintext</code>  protocols, which Java improves performance and stability compared to the JNLP3 protocol.
+New <code>JNLP4-connect</code> protocol, which improves performance and stability compared to the JNLP3 protocol.
 (https://github.com/jenkinsci/remoting/pull/92)
 * [JENKINS-37565](https://issues.jenkins-ci.org/browse/JENKINS-37565) - 
 Update the required Java version to Java 7, Java 6 is not supported anymore.

docs/protocols.md

@@ -53,12 +53,14 @@ Protocol uses non-blocking I/O wherever possible which removes the performance b
 ### JNLP4-plaintext
 
 * Introduced in: Remoting 3.0, [JENKINS-36871](https://issues.jenkins-ci.org/browse/JENKINS-36871)
-* Use with caution
+* For performance testing **only**, not supported for other purposes
+* Cannot be used in Jenkins
 
-The protocol is similar to <code>JNLP4-plaintext</code>, 
-  but it does not setup the TLS encryption between agent and master.
-In particular cases it may expose the secret data.
+This protocol was developed to allow for like for like performance comparison 
+  between the original NIO engine used by <code>JNLP2-connect</code> and the new NIO engine 
+  used by <connect>JNLP4-connect</code>. 
 
-On the other had, the protocol allows to get extra performance in the case when the remoting layer does not need its own encryption   
-  (e.g. when the connection goes within a secured VPN).
+The protocol is similar to <code>JNLP4-connect</code>, 
+  but it does not setup the TLS encryption between agent and master.
+As this protocol is plaintext it is not for use outside of like for like performance testing.