Add missing CSS modules API

[posva]

When using the options api, users can directly read from this.$style to use CSS modules, when using setup, they have to use useCssModules() and useCssVars(). These two functions seems to be missing from API docs

[skirtles-code]

useCssModule is now documented: https://v3.vuejs.org/api/global-api.html#usecssmodule

I'm unclear whether useCssVars is part of the public API. It seems to be related to the v-bind-in-<style> feature but from what I can tell from the RFC it's only intended for internal use by the SFC compiler.

@posva Do you think this can be closed?