Document v-bind.attr shorthand in API section. (#1571)

* Document v-bind.attr shorthand in API section.

* Update src/api/built-in-directives.md

Co-authored-by: skirtle <[email protected]>

Co-authored-by: skirtle <[email protected]>

Author: LinusBorg

src/api/built-in-directives.md

@@ -319,13 +319,16 @@ Dynamically bind one or more attributes, or a component prop to an expression.
   <svg><a :xlink:special="foo"></a></svg>
   ```
 
-  The `.prop` modifier also has a dedicated shorthand, `.`:
+  The `.prop` and `.attr` modifiers also have a dedicated shorthand, `.` and `^` respectively:
 
   ```vue-html
   <div :someProperty.prop="someObject"></div>
-
   <!-- equivalent to -->
   <div .someProperty="someObject"></div>
+  
+  <div :someProperty.attr="someString"></div>
+  <!-- equivalent to -->
+  <div ^someProperty="someString"></div>
   ```
 
   The `.camel` modifier allows camelizing a `v-bind` attribute name when using in-DOM templates, e.g. the SVG `viewBox` attribute: