Accept elements as the tooltip / popover content

When a DOM node is passed to an HTML tooltip, the `title` node is only
moved if it is not already in the tooltip. Otherwise, `empty()` is used
instead of `detach()` before appending the `title` to avoid memory
leaks. If a DOM node is passed to a plain text tooltip, its text is
copied via jQuery `.text()`.

Replaces `.detach()` with `.empty()`, as `.detach()` is almost never
useful but instead leaks memory. The difference between `empty` and
`detach` is that the latter keeps all the attached jQuery events/data.
However, since we do not return the previous children, the user would
have to keep these themselves, thus they can `detach()` if necessary.

This is a port of https://github.com/twbs/bootstrap/pull/14552 to v4.

docs/components/popovers.md

@@ -193,7 +193,7 @@ Options can be passed via data attributes or JavaScript. For data attributes, ap
       </tr>
       <tr>
         <td>content</td>
-        <td>string | function</td>
+        <td>string | element | function</td>
         <td>''</td>
         <td>
           <p>Default content value if <code>data-content</code> attribute isn't present.</p>
@@ -245,7 +245,7 @@ Options can be passed via data attributes or JavaScript. For data attributes, ap
       </tr>
       <tr>
         <td>title</td>
-        <td>string | function</td>
+        <td>string | element | function</td>
         <td>''</td>
         <td>
           <p>Default title value if <code>title</code> attribute isn't present.</p>

docs/components/tooltips.md

@@ -203,7 +203,7 @@ Options can be passed via data attributes or JavaScript. For data attributes, ap
       </tr>
       <tr>
         <td>title</td>
-        <td>string | function</td>
+        <td>string | element | function</td>
         <td>''</td>
         <td>
           <p>Default title value if <code>title</code> attribute isn't present.</p>