Add snippet generator link to help page

Users frequently comment in the feedback on the jenkins.io checkout
page https://www.jenkins.io/doc/pipeline/steps/workflow-scm-step/ that
they would like more examples.

Many of those writing the comments may not be aware that the Pipeline
Snippet Generator can provide help from within their Jenkins server
for the specific plugins and plugin versions they are using.

Hyperlink to the Pipeline Snippet Generator in hopes that users will
see one of the hyperlinks and understand that they can use their own
Jenkins server for better information about the Pipeline Syntax.

Author: MarkEWaite

src/main/resources/org/jenkinsci/plugins/workflow/steps/scm/GenericSCMStep/help.jelly

@@ -2,11 +2,13 @@
 <?jelly escape-by-default='true'?>
 <div xmlns:j="jelly:core" xmlns:st="jelly:stapler">
     <p>
-    This is a special step that allows to run checkouts using any configuration
+    This is a special step that will checkout using the configuration
     options offered by any Pipeline-compatible SCM plugin.
-    To use a concrete SCM implementations, just install the corresponding plugin 
-    and check if it is shown in the list below.
-    Then select the SCM to use from the dropdown list and configure it as needed.
+    Install the corresponding SCM plugin and select it from the list in the
+    <strong><a href="https://jenkins.io/doc/book/pipeline/getting-started/#snippet-generator">Pipeline Snippet Generator</a?</strong>.
+    The
+    <strong><a href="https://jenkins.io/doc/book/pipeline/getting-started/#snippet-generator">Pipeline Snippet Generator</a?</strong>
+    uses the installed SCM plugins of your Jenkins server to suggest pipeline snippets.
     </p>
     <p>This step returns a <code>Map</code> of any variables the SCM plugin would
     set in a Freestyle job, so if your SCM is git, you can do: