Add help text for RunWrapper 'externalizableId' property

Author: stuartrowe

src/main/resources/org/jenkinsci/plugins/workflow/support/steps/build/RunWrapper/help.html

@@ -12,6 +12,7 @@
     <dt><code>fullProjectName</code></dt><dd>Full name of the project of this build, including folders such as <code>folder1/folder2/foo</code>.</dd>
     <dt><code>description</code></dt><dd>additional information about the build</dd>
     <dt><code>id</code></dt><dd>normally <code>number</code> as a string</dd>
+    <dt><code>externalizableId</code></dt><dd>identifier for this build from combining <code>fullProjectName</code> and <code>number</code> as <code>fullProjectName#number</code></dd>
     <dt><code>timeInMillis</code></dt><dd>time since the epoch when the build was scheduled</dd>
     <dt><code>startTimeInMillis</code></dt><dd>time since the epoch when the build started running</dd>
     <dt><code>duration</code></dt><dd>duration of the build in milliseconds</dd>