docs(next): Update config comment (#4664)

* Update Custom JWT decode method
* Update docs/docs/configuration/nextjs.md

Co-authored-by: Lluis Agusti <hi@llu.lu>

docs/docs/configuration/nextjs.md

@@ -34,8 +34,8 @@ import jwt from "jsonwebtoken";
 
 export default NextAuth({
   providers: [...],
+  secret: /* Please use `process.env.NEXTAUTH_SECRET` */,
   jwt: {
-    // secret: PLEASE USE process.env.NEXTAUTH_SECRET
     encode: async ({ secret, token }) => {
       return jwt.sign(token as any, secret);
     },