If JenkinsLocationConfiguration.jenkinsUrl is still null,...

If JenkinsLocationConfiguration.jenkinsUrl is still null, CoreEnvironmentContributor will silently skip several variables.
No obvious fix for that, but at least document it so it is not so surprising.

core/src/main/resources/jenkins/model/CoreEnvironmentContributor/buildEnv.properties

@@ -12,6 +12,6 @@ NODE_NAME.blurb=Name of the slave if the build is on a slave, or "master" if run
 NODE_LABELS.blurb=Whitespace-separated list of labels that the node is assigned.
 WORKSPACE.blurb=The absolute path of the directory assigned to the build as a workspace.
 JENKINS_HOME.blurb=The absolute path of the directory assigned on the master node for Jenkins to store data.
-JENKINS_URL.blurb=Full URL of Jenkins, like <tt>http://server:port/jenkins/</tt>
-BUILD_URL.blurb=Full URL of this build, like <tt>http://server:port/jenkins/job/foo/15/</tt>
-JOB_URL.blurb=Full URL of this job, like <tt>http://server:port/jenkins/job/foo/</tt>
+JENKINS_URL.blurb=Full URL of Jenkins, like <tt>http://server:port/jenkins/</tt> (note: only available if <i>Jenkins URL</i> set in system configuration)
+BUILD_URL.blurb=Full URL of this build, like <tt>http://server:port/jenkins/job/foo/15/</tt> (<i>Jenkins URL</i> must be set)
+JOB_URL.blurb=Full URL of this job, like <tt>http://server:port/jenkins/job/foo/</tt> (<i>Jenkins URL</i> must be set)