Add doc for `.attr` and `.prop` modifier usage in render function.

[leifmarcus]

As mentioned in an issue on Vue core, I wanted to find more information about how to add modifiers for props in render functions: vuejs/core#5647.

As @LinusBorg suggested, I create an issue here. I suggest to add an example for the modifiers to this page: https://vuejs.org/api/render-function.html