Document that .form-inline & .form-horizontal work fine on non-<form>s too

Fixes #15555
[skip sauce]

docs/_includes/css/forms.html

@@ -56,7 +56,7 @@
 
 
   <h2 id="forms-inline">Inline form</h2>
-  <p>Add <code>.form-inline</code> to your <code>&lt;form&gt;</code> for left-aligned and inline-block controls. <strong>This only applies to forms within viewports that are at least 768px wide.</strong></p>
+  <p>Add <code>.form-inline</code> to your form (which doesn't have to be a <code>&lt;form&gt;</code>) for left-aligned and inline-block controls. <strong>This only applies to forms within viewports that are at least 768px wide.</strong></p>
   <div class="bs-callout bs-callout-danger" id="callout-inline-form-width">
     <h4>Requires custom widths</h4>
     <p>Inputs and selects have <code>width: 100%;</code> applied by default in Bootstrap. Within inline forms, we reset that to <code>width: auto;</code> so multiple controls can reside on the same line. Depending on your layout, additional custom widths may be required.</p>
@@ -130,7 +130,7 @@
 {% endhighlight %}
 
   <h2 id="forms-horizontal">Horizontal form</h2>
-  <p>Use Bootstrap's predefined grid classes to align labels and groups of form controls in a horizontal layout by adding <code>.form-horizontal</code> to the form. Doing so changes <code>.form-group</code>s to behave as grid rows, so no need for <code>.row</code>.</p>
+  <p>Use Bootstrap's predefined grid classes to align labels and groups of form controls in a horizontal layout by adding <code>.form-horizontal</code> to the form (which doesn't have to be a <code>&lt;form&gt;</code>). Doing so changes <code>.form-group</code>s to behave as grid rows, so no need for <code>.row</code>.</p>
   <div class="bs-example" data-example-id="simple-horizontal-form">
     <form class="form-horizontal">
       <div class="form-group">