Merge pull request #496 from daniel-beck/upgrade-guide-2.19.4

Add upgrade guide for 2.19.4

content/doc/upgrade-guide/2.19.adoc

@@ -8,6 +8,44 @@ section: doc
 
 Each section covers the upgrade from the previous LTS release, the section on 2.19.1 covers the upgrade from 2.7.4.
 
+=== Upgrading to Jenkins LTS 2.19.4
+
+==== JNLP agent protocol options weren't correctly persisted
+
+https://issues.jenkins-ci.org/browse/JENKINS-39465[JENKINS-39465]
+
+The JNLP agent protocol configuration could not be loaded from disk after a Jenkins restart.
+
+If you customized these settings since 2.19.1, review that they are configured as expected.
+
+==== Honor `jenkins.model.Jenkins.slaveAgentPort` again
+
+https://issues.jenkins-ci.org/browse/JENKINS-38187[JENKINS-38187]
+
+A regression in Jenkins 2.0 caused Jenkins to ignore the System property `jenkins.model.Jenkins.slaveAgentPort`.
+
+The previous behavior has been restored, and a new System property `jenkins.model.Jenkins.slaveAgentPort` added that, if set, will enforce the JNLP agent port on every Jenkins startup, and will not allow the value to be changed through the UI.
+
+It can still be changed via Groovy scripting for troubleshooting purposes, but any change will be reverted on startup.
+
+==== Text boxes on configuration screens will no longer stretch to accomodate long lines
+
+https://issues.jenkins-ci.org/browse/JENKINS-27367[JENKINS-27367]
+
+The layout of job configuration screens will no longer break if users don't insert extra line breaks to break long lines. Instead, the text box will allow horizontal scrolling.
+
+==== Don't spawn numerous threads on the agent build history page
+
+https://issues.jenkins-ci.org/browse/JENKINS-23244[JENKINS-23244]
+
+The build history for specific agents has long caused numerous potentially long-running threads to spawn. This has been fixed.
+
+==== Memory leak on dashboard fixed
+
+https://issues.jenkins-ci.org/browse/JENKINS-10912[JENKINS-10912]
+
+A memory leak on the dashboard (view pages) was fixed, so that browser memory use will no longer grow unbounded.
+
 === Upgrading to Jenkins LTS 2.19.3
 
 ==== Expansion of the serialization blacklist