Document caveats of `pointer-events: none` usage in btn.disabled

Refs #10049
Source for compatibility info: http://caniuse.com/#feat=pointer-events

docs/css.html

@@ -1886,8 +1886,8 @@ For example, <code>&lt;section&gt;</code> should be wrapped as inline.
   <p>Add the <code>disabled</code> attribute to a <code>&lt;fieldset&gt;</code> to disable all the controls within the <code>&lt;fieldset&gt;</code> at once.</p>
 
   <div class="bs-callout bs-callout-warning">
-    <h4>Link functionality of <code>&lt;a&gt;</code> not impacted</h4>
-    <p>This class will only change the appearance of <code>&lt;a class="btn btn-default"&gt;</code> buttons, not their functionality. Use custom JavaScript to disable links here.</p>
+    <h4>Caveat about link functionality of <code>&lt;a&gt;</code></h4>
+    <p>Our styles use <code>pointer-events: none</code> to try to disable the link functionality of <code>&lt;a class="btn btn-*"&gt;</code> buttons in this case, but that CSS property is not yet standardized and isn't fully supported in Opera 18 and below, or in Internet Explorer 11. So to be safe, use custom JavaScript to disable such links.</p>
   </div>
 
   <div class="bs-callout bs-callout-danger">
@@ -2292,8 +2292,8 @@ For example, <code>&lt;section&gt;</code> should be wrapped as inline.
     We use <code>.disabled</code> as a utility class here, similar to the common <code>.active</code> class, so no prefix is required.
   </p>
   <div class="bs-callout bs-callout-warning">
-    <h4>Link functionality not impacted</h4>
-    <p>This class will only change the <code>&lt;a&gt;</code>'s appearance, not its functionality. Use custom JavaScript to disable links here.</p>
+    <h4>Link functionality caveat</h4>
+    <p>This class uses <code>pointer-events: none</code> to try to disable the link functionality of <code>&lt;a&gt;</code>s, but that CSS property is not yet standardized and isn't fully supported in Opera 18 and below, or in Internet Explorer 11. So to be safe, use custom JavaScript to disable such links.</p>
   </div>
   <div class="bs-callout bs-callout-warning">
     <h4>Context-specific usage</h4>