[SPARK-29453][WEBUI] Improve tooltips information for SQL tab

### What changes were proposed in this pull request?
Adding tooltip to SQL tab for better usability.

### Why are the changes needed?
There are a few common points of confusion in the UI that could be clarified with tooltips. We
 should add tooltips to explain.

### Does this PR introduce any user-facing change?
yes.
![Screenshot 2019-11-23 at 9 47 41 AM](https://user-images.githubusercontent.com/8948111/69472963-aaec5980-0dd6-11ea-881a-fe6266171054.png)

### How was this patch tested?
Manual test.

Closes apache#26641 from 07ARB/SPARK-29453.

Authored-by: 07ARB <[email protected]>
Signed-off-by: Sean Owen <[email protected]>

Author: 07ARB

sql/core/src/main/scala/org/apache/spark/sql/execution/ui/AllExecutionsPage.scala

@@ -324,7 +324,15 @@ private[ui] class ExecutionPagedTable(
 
             <th>
               <a href={headerLink}>
-                {header}
+                {if (header == "Duration") {
+                  <span data-toggle="tooltip" data-placement="top"
+                    title="Time from query submission to completion
+                    (or if still executing, time since submission)">
+                    {header}
+                  </span>
+                } else {
+                  {header}
+                }}
               </a>
             </th>
           } else {