chore(button): add known issue for tooltip/popover

* add known issue to button docs indicating a CSS issue when using a tooltip or
  a popover.

src/buttons/docs/readme.md

@@ -47,4 +47,4 @@ With the buttons directive, we can make a group of buttons behave like a set of
 
 ### Known issues
 
-To use tooltips or popovers on a button group, make sure you add the applicable tooltip or popover directive directly to your `btn-group` element and set `appendToBody` to `true`. This is due to Bootstrap CSS styling. See [here](http://getbootstrap.com/components/#btn-groups) for more information.
+To use tooltips or popovers on an element inside a button group, make sure you add the applicable tooltip or popover directive directly to your `btn-group` element and set `appendToBody` to `true`. This is due to Bootstrap CSS styling. See [here](http://getbootstrap.com/components/#btn-groups) for more information.