Add initialization examples to tooltip and popover "Opt-in functionality" callouts

Fixes #14950.
Closes #15015 by merging it.

docs/_includes/js/popovers.html

@@ -12,6 +12,12 @@
   <div class="bs-callout bs-callout-danger">
     <h4>Opt-in functionality</h4>
     <p>For performance reasons, the Tooltip and Popover data-apis are opt-in, meaning <strong>you must initialize them yourself</strong>.</p>
+    <p>One way to initialize all popovers on a page would be to select them by their <code>data-toggle</code> attribute:</p>
+{% highlight js %}
+$(function () {
+  $('[data-toggle="popover"]').popover()
+})
+{% endhighlight %}
   </div>
   <div class="bs-callout bs-callout-warning">
     <h4>Popovers in button groups and input groups require special setting</h4>

docs/_includes/js/tooltips.html

@@ -61,6 +61,13 @@
   <div class="bs-callout bs-callout-danger">
     <h4>Opt-in functionality</h4>
     <p>For performance reasons, the Tooltip and Popover data-apis are opt-in, meaning <strong>you must initialize them yourself</strong>.</p>
+    <p>One way to initialize all tooltips on a page would be to select them by their <code>data-toggle</code> attribute:</p>
+{% highlight js %}
+$(function () {
+  $('[data-toggle="tooltip"]').tooltip()
+})
+{% endhighlight %}
+
   </div>
   <div class="bs-callout bs-callout-warning">
     <h4>Tooltips in button groups and input groups require special setting</h4>