Add missing arbitrary value option to flex API table (#2002)

* Add missing arbitrary value option to `flex` API table

* Update flex.mdx

---------

Co-authored-by: Adam Wathan <adam.wathan@gmail.com>

Author: dcastil

src/docs/flex.mdx

@@ -14,6 +14,8 @@ export const description = "Utilities for controlling how flex items both grow a
     ["flex-auto", "flex: 1 1 auto;"],
     ["flex-initial", "flex: 0 1 auto;"],
     ["flex-none", "flex: none;"],
+    ["flex-[<value>]", "flex: <value>;"],
+    ["flex-[<custom-property>]", "flex: var(<custom-property>);"]
   ]}
 />