Document arguments passed to `placement` function of tooltip/popover

Closes #14255

docs/_includes/js/popovers.html

@@ -187,7 +187,10 @@ sagittis lacus vel augue laoreet rutrum faucibus.">
           <td>placement</td>
           <td>string | function</td>
           <td>'right'</td>
-          <td>How to position the popover - top | bottom | left | right | auto.<br> When "auto" is specified, it will dynamically reorient the popover. For example, if placement is "auto left", the popover will display to the left when possible, otherwise it will display right.</td>
+          <td>
+            <p>How to position the popover - top | bottom | left | right | auto.<br>When "auto" is specified, it will dynamically reorient the popover. For example, if placement is "auto left", the popover will display to the left when possible, otherwise it will display right.</p>
+            <p>When a function is used to determine the placement, it is called with the popover DOM node as its first argument and the triggering element DOM node as its second. The <code>this</code> context is set to the popover instance.</p>
+          </td>
         </tr>
         <tr>
           <td>selector</td>

docs/_includes/js/tooltips.html

@@ -119,7 +119,10 @@ $('#example').tooltip(options)
           <td>placement</td>
           <td>string | function</td>
           <td>'top'</td>
-         <td>How to position the tooltip - top | bottom | left | right | auto. <br> When "auto" is specified, it will dynamically reorient the tooltip. For example, if placement is "auto left", the tooltip will display to the left when possible, otherwise it will display right.</td>
+          <td>
+            <p>How to position the tooltip - top | bottom | left | right | auto.<br>When "auto" is specified, it will dynamically reorient the tooltip. For example, if placement is "auto left", the tooltip will display to the left when possible, otherwise it will display right.</p>
+            <p>When a function is used to determine the placement, it is called with the tooltip DOM node as its first argument and the triggering element DOM node as its second. The <code>this</code> context is set to the tooltip instance.</p>
+          </td>
         </tr>
         <tr>
           <td>selector</td>