mark up variables (#4064)

* mark up variables

* mark up variables

core/src/main/resources/hudson/PluginWrapper/uninstall.properties

-msg=You are about to uninstall the <b>{0}</b> plugin. This will remove the plugin binary from your $JENKINS_HOME, \
+msg=You are about to uninstall the <b>{0}</b> plugin. This will remove the plugin binary from your <tt>$JENKINS_HOME</tt>, \
   but it will leave the configuration files of the plugin untouched.
 title=Uninstalling {0} plugin

war/src/main/webapp/help/system-config/globalEnvironmentVariables.html

 <div>
     These key-value pairs apply for every build on every node. They can be used in Jenkins'
-    configuration (as $key or ${key}) and will be added to the environment for processes launched from the build.
+    configuration (as <tt>$key</tt> or <tt>${key}</tt>) and will be added to the environment for processes launched from the build.
 </div>