docs: note about new props when using props: true (#974)

* Clarify props must match param names

* added comment to code example

* Update docs/guide/essentials/passing-props.md

Co-authored-by: Eduardo San Martin Morote <posva@users.noreply.github.com>

docs/guide/essentials/passing-props.md

@@ -15,6 +15,7 @@ with
 
 ```js
 const User = {
+  // make sure to add a prop named exactly like the route param
   props: ['id'],
   template: '<div>User {{ id }}</div>'
 }