docs: add description of babel plugin usage in TS plugin prompt (#4046)

closes #3206

(cherry picked from commit 82dbbeb)

Author: haoqunjiang

packages/@vue/cli-plugin-typescript/prompts.js

@@ -13,7 +13,7 @@ const prompts = module.exports = [
   {
     name: `useTsWithBabel`,
     type: `confirm`,
-    message: `Use Babel alongside TypeScript for auto-detected polyfills?`
+    message: 'Use Babel alongside TypeScript (required for modern mode, auto-detected polyfills, transpiling JSX)?'
   },
   {
     name: `lint`,

packages/@vue/cli/lib/promptModules/typescript.js

@@ -22,7 +22,7 @@ module.exports = cli => {
     name: 'useTsWithBabel',
     when: answers => answers.features.includes('ts'),
     type: 'confirm',
-    message: 'Use Babel alongside TypeScript for auto-detected polyfills?',
+    message: 'Use Babel alongside TypeScript (required for modern mode, auto-detected polyfills, transpiling JSX)?',
     description: 'It will output ES2015 and delegate the rest to Babel for auto polyfill based on browser targets.',
     default: answers => answers.features.includes('babel')
   })