Improves custom message help text

src/main/webapp/help-projectConfig-slackCustomMessage.html

@@ -1,5 +1,5 @@
 <div>
   <p>
-    Enter a custom message that will be included with the notifications. Include build variables in the form <em>$VAR_NAME</em>.
+    Enter a custom message that will be included with the notifications. Include build variables in the form <a href="https://wiki.jenkins-ci.org/display/JENKINS/Building+a+software+project#Buildingasoftwareproject-JenkinsSetEnvironmentVariables">Jenkins Environment Variables</a> by putting <em>$VAR_NAME</em> where you want it to appear.
   </p>
 </div>