docs(textarea): add corrected guidelines link and rootClass (#3326)

- adds a spectrum-Textfield--multiline rootClass to textarea component
- adds an additional spectrum object to contain the text area guidance
link in package.json
- remove duplicating custom class

Author: marissahuysentruyt

components/textfield/stories/textarea.stories.js

@@ -18,6 +18,7 @@ export default {
 	},
 	args: {
 		...Textfield.args,
+		rootClass: "spectrum-Textfield--multiline",
 		labelText: "Comments"
 	},
 	parameters: {

components/textfield/stories/textarea.template.js

@@ -4,14 +4,13 @@ import { Template as Textfield } from "./template";
 
 export const Template = ({
 	customClasses = [],
-	rootClass = "spectrum-Textfield",
+	rootClass = "spectrum-Textfield--multiline",
 	size = "m",
 	multiline = true,
 	...item
 } = {}, context = {}) => Textfield({
 	customClasses: [
 		rootClass,
-		typeof size !== "undefined" ? `${rootClass}--size${size.toUpperCase()}` : null,
 		...customClasses
 	],
 	size,