Add helptext for checkPrecondition expression

Added helptext which points to Spring Expression Language so that users understand they can make complex evaluations here.

app/scripts/modules/core/help/helpContents.js

@@ -111,6 +111,8 @@ module.exports = angular.module('spinnaker.core.help.contents', [])
     'pipeline.config.checkPreconditions.failPipeline': '' +
       '<p><strong>Checked</strong> - the overall pipeline will fail whenever this precondition is false.</p>' +
       '<p><strong>Unchecked</strong> - the overall pipeline will continue executing but this particular branch will stop.</p>',
+    'pipeline.config.checkPreconditions.expression': '<p>Value must evaluate to "true".' +
+      ' Use of the <b>Spring Expression Language</b> within the placeholder ${ } allows for complex evaluations.</p>',
     'pipeline.config.deploy.template': '<p>Select an existing cluster to use as a template for this deployment, and we\'ll pre-fill ' +
       'the configuration based on the newest server group in the cluster.</p>' +
       '<p>If you want to start from scratch, select "None".</p>' +

app/scripts/modules/core/pipeline/config/preconditions/types/expression/additionalFields.html

@@ -1,6 +1,6 @@
 <ng-form name="expressionForm">
     <div class="form-group row">
-        <div class="col-sm-3 sm-label-left">Expression</div>
+        <div class="col-sm-3 sm-label-left">Expression <help-field key="pipeline.config.checkPreconditions.expression"></help-field></div>
         <div class="col-sm-9">
             <input type="text"
                    name="expression"