docs(select): add known issue about Firefox selection behavior

Related to angular#9134

Author: Narretz

src/ng/directive/select.js

@@ -382,6 +382,19 @@ var SelectController =
  * @param {string=} ngAttrSize sets the size of the select element dynamically. Uses the
  * {@link guide/interpolation#-ngattr-for-binding-to-arbitrary-attributes ngAttr} directive.
  *
+ *
+ * @knownIssue
+ *
+ * In Firefox, the select model is only updated when the select element is blurred. For example,
+ * when switching between options with the keyboard, the select model is only set to the
+ * currently selected option when the select is blurred, e.g via tab key or clicking the mouse
+ * outside the select.
+ *
+ * This is due to an ambiguity in the select element specification. See the
+ * [issue on the Firefox bug tracker](https://bugzilla.mozilla.org/show_bug.cgi?id=126379)
+ * for more information, and this
+ * [Github comment for a workaround](https://github.com/angular/angular.js/issues/9134#issuecomment-130800488)
+ *
  * @example
  * ### Simple `select` elements with static options
  *