doc: add styleText hex color doc

Author: araujogui

doc/api/util.md

@@ -2548,7 +2548,8 @@ changes:
 -->
 
 * `format` {string | Array} A text format or an Array
-  of text formats defined in `util.inspect.colors`.
+  of text formats defined in `util.inspect.colors`, or a hex color in `#RGB`
+  or `#RRGGBB` form.
 * `text` {string} The text to to be formatted.
 * `options` {Object}
   * `validateStream` {boolean} When true, `stream` is checked to see if it can handle colors. **Default:** `true`.