Add information on how to replace default clearable icon

[mesqueeb]

I'm only using the ionicons icon set in many Quasar apps. I always stumble on icons used by Quasar components like the form clearable icon that do not get rendered because the material icon set is not used.

We should add some information on any mention of such icons in the documentation, how to set them to another icon in case a different icon set was chosen.