[SPARK-29460][WEBUI] Add tooltip for Jobs page

### What changes were proposed in this pull request?
Adding tooltip for jobs tab column - Job Id (Job Group), Description ,Submitted, Duration, Stages, Tasks

Before:
![Screenshot from 2019-11-04 11-31-02](https://user-images.githubusercontent.com/51401130/68102467-e8a54300-fef8-11e9-9f9e-48dd1b393ac8.png)

After:
![Screenshot from 2019-11-04 11-30-53](https://user-images.githubusercontent.com/51401130/68102478-f3f86e80-fef8-11e9-921a-357678229cb4.png)

### Why are the changes needed?
Jobs tab do not have any tooltip for the columns, Some page provide tooltip , inorder to resolve the inconsistency and for better user experience.

### Does this PR introduce any user-facing change?
No

### How was this patch tested?
Manual

Closes #26384 from PavithraRamachandran/jobTab_tooltip.

Authored-by: Pavithra Ramachandran <pavi.rams@gmail.com>
Signed-off-by: Sean Owen <srowen@gmail.com>

core/src/main/scala/org/apache/spark/ui/jobs/AllJobsPage.scala

@@ -541,20 +541,23 @@ private[ui] class JobPagedTable(
 
   override def headers: Seq[Node] = {
     // Information for each header: title, cssClass, and sortable
-    val jobHeadersAndCssClasses: Seq[(String, String, Boolean)] =
+    val jobHeadersAndCssClasses: Seq[(String, String, Boolean, Option[String])] =
       Seq(
-        (jobIdTitle, "", true),
-        ("Description", "", true), ("Submitted", "", true), ("Duration", "", true),
-        ("Stages: Succeeded/Total", "", false),
-        ("Tasks (for all stages): Succeeded/Total", "", false)
+        (jobIdTitle, "", true, None),
+        ("Description", "", true, None),
+        ("Submitted", "", true, None),
+        ("Duration", "", true, Some("Elapsed time since the job was submitted " +
+          "until execution completion of all its stages.")),
+        ("Stages: Succeeded/Total", "", false, None),
+        ("Tasks (for all stages): Succeeded/Total", "", false, None)
       )
 
     if (!jobHeadersAndCssClasses.filter(_._3).map(_._1).contains(sortColumn)) {
       throw new IllegalArgumentException(s"Unknown column: $sortColumn")
     }
 
     val headerRow: Seq[Node] = {
-      jobHeadersAndCssClasses.map { case (header, cssClass, sortable) =>
+      jobHeadersAndCssClasses.map { case (header, cssClass, sortable, tooltip) =>
         if (header == sortColumn) {
           val headerLink = Unparsed(
             parameterPath +
@@ -566,9 +569,17 @@ private[ui] class JobPagedTable(
 
           <th class={cssClass}>
             <a href={headerLink}>
-              {header}<span>
-              &nbsp;{Unparsed(arrow)}
-            </span>
+              {
+                if (tooltip.nonEmpty) {
+                  <span data-toggle="tooltip" data-placement="top" title={tooltip.get}>
+                    {header}&nbsp;{Unparsed(arrow)}
+                  </span>
+                } else {
+                  <span>
+                    {header}&nbsp;{Unparsed(arrow)}
+                  </span>
+                }
+              }
             </a>
           </th>
         } else {
@@ -581,12 +592,32 @@ private[ui] class JobPagedTable(
 
             <th class={cssClass}>
               <a href={headerLink}>
-                {header}
-              </a>
+                {
+                  if (tooltip.nonEmpty) {
+                    <span data-toggle="tooltip" data-placement="top" title={tooltip.get}>
+                      {header}
+                    </span>
+                  } else {
+                    <span>
+                      {header}
+                    </span>
+                  }
+                }
+               </a>
             </th>
           } else {
             <th class={cssClass}>
-              {header}
+              {
+                if (tooltip.nonEmpty) {
+                  <span data-toggle="tooltip" data-placement="top" title={tooltip.get}>
+                    {header}
+                  </span>
+                } else {
+                  <span>
+                    {header}
+                  </span>
+                }
+              }
             </th>
           }
         }