[JENKINS-38640] [JENKINS-36914] Warn users to avoid stash/unstash of …

…large files

src/main/resources/org/jenkinsci/plugins/workflow/support/steps/stash/StashStep/help.html

@@ -1,4 +1,7 @@
 <div>
     Saves a set of files for use later in the same build, generally on another node/workspace.
     Stashed files are not otherwise available and are generally discarded at the end of the build.
+    <strong>Note</strong> that the <code>stash</code> and <code>unstash</code> steps are designed for use with small files.
+    For large data transfers, use the External Workspace Manager plugin,
+    or use an external repository manager such as Nexus or Artifactory.
 </div>