Add info to docs on how to prevent using incorrect twMerge

[dcastil]

When configuring tailwind-merge, one creates their own twMerge function to call. I should add a guide on how to prevent to accidentally use import { twMerge } from 'tailwind-merge' instead of import { twMerge } from 'src/my-own-tailwind-merge' and explain the ESLint rule I use for that.