Update border-tokens.en.mdx (tokens-studio#60)

Author: six7

pages/available-tokens/border-tokens.en.mdx

@@ -4,3 +4,7 @@ You can create Border tokens to define border styles. The token must contain the
 - Color: the color of the border
 - Width: the width or thickness of the border. Can be defined either unitless or with a unit such as px or rem.
 - Style: the border style. Currently solid and dashed is supported.
+
+import Callout from 'nextra-theme-docs/callout'
+
+<Callout>This token will be applied under the `border` property, which was previously used by border color tokens. The plugin will automatically migrate any older applied color tokens under the `border` property to the new `borderColor` one.</Callout>