docs: note about existing content at same path on different branches

vintro

2023-08-14 f1c9ca495e450ecb62dade70c4b60d86e106f79c

 quartz/components/MobileOnly.tsx

@@ -4,9 +4,7 @@
  if (component) {
    const Component = component
    function MobileOnly(props: QuartzComponentProps) {
      return <div class="mobile-only">
        <Component {...props} />
      </div>
      return <Component displayClass="mobile-only" {...props} />
    }

    MobileOnly.displayName = component.displayName