add hover hint for auto-enroll

lms/static/sass/course/instructor/_instructor_2.scss

@@ -189,6 +189,23 @@
     cursor: pointer;
   }
 
+  label[for="auto-enroll"]:hover + .auto-enroll-hint {
+    display: block;
+  }
+
+  .auto-enroll-hint {
+    display: none;
+    background-color: $white;
+    border: 1px solid $light-gray;
+    width: $baseline * 25;
+    padding: $baseline;
+    position: absolute;
+
+    span.emph {
+      font-weight: bold;
+    }
+  }
+
   .batch-enrollment {
     textarea {
       margin-top: 0.2em;

lms/templates/courseware/instructor_dashboard_2/membership.html

@@ -9,6 +9,12 @@
   <input type="button" name="unenroll" value="Unenroll" data-endpoint="${ section_data['unenroll_button_url'] }" >
   <input type="checkbox" name="auto-enroll" value="Auto-Enroll" style="margin-top: 1em;">
   <label for="auto-enroll">Auto Enroll</label>
+  <div class="auto-enroll-hint">
+    <p> If auto enroll is <span class="emph">checked</span>, students who have not yet registered for edX will be automatically enrolled.
+        If auto enroll is left <span class="emph">unchecked</span>, students who have not yet registered for edX will not be enrolled,
+        but will be allowed to enroll.
+    </p>
+  </div>
   <div class="request-response"></div>
   <div class="request-response-error"></div>
 </div>