[SPARK-29476][WEBUI] add tooltip for Thread

### What changes were proposed in this pull request?
Adding tooltip for Thread Dump - Thread Locks

Before:
![Screenshot from 2019-11-04 17-11-22](https://user-images.githubusercontent.com/51401130/68127349-b963f580-ff3b-11e9-8547-e01907382632.png)

After:
![Screenshot from 2019-11-13 18-12-54](https://user-images.githubusercontent.com/51401130/68768698-08e7a700-0649-11ea-804b-2eb4d5f162b4.png)

### Why are the changes needed?
Thread Dump 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 #26386 from PavithraRamachandran/threadDump_tooltip.

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

core/src/main/scala/org/apache/spark/ui/exec/ExecutorThreadDumpPage.scala

@@ -89,7 +89,12 @@ private[ui] class ExecutorThreadDumpPage(
           <th onClick="collapseAllThreadStackTrace(false)">Thread ID</th>
           <th onClick="collapseAllThreadStackTrace(false)">Thread Name</th>
           <th onClick="collapseAllThreadStackTrace(false)">Thread State</th>
-          <th onClick="collapseAllThreadStackTrace(false)">Thread Locks</th>
+          <th onClick="collapseAllThreadStackTrace(false)">
+            <span data-toggle="tooltip" data-placement="top"
+                  title="Objects whose lock the thread currently holds">
+              Thread Locks
+            </span>
+          </th>
         </thead>
         <tbody>{dumpRows}</tbody>
       </table>