message: docs: clarify certificateArn input
diff: @@ -437,7 +437,7 @@ The fourth cache behaviour handles next API requests `api/*`.
  | build.enabled | `boolean` | `true` | Same as passing `build:false` but from within the config |
  | build.env | `object` | `{}` | Add additional environment variables to the script |
  | cloudfront | `object` | `{}` | Inputs to be passed to [aws-cloudfront](https://github.com/serverless-components/aws-cloudfront) |
  -| certificateArn | `string` | `` | Specific certificate ARN to use for CloudFront distribution. Helpful if you have a wildcard SSL cert you wish to use. |
  +| certificateArn | `string` | `` | Specific certificate ARN to use for CloudFront distribution. Helpful if you have a wildcard SSL cert you wish to use. This currently works only in tandem with the `domain` input. Please check [custom CloudFront configuration](https://github.com/serverless-nextjs/serverless-next.js#custom-cloudfront-configuration) for how to specify `certificate` without needing to use the `domain` input (note that doing so will override any certificate due to the domain input). |
  | domainType | `string` | `"both"` | Can be one of: `"apex"` - apex domain only, don't create a www subdomain. `"www"` - www domain only, don't create an apex subdomain.`"both"` - create both www and apex domains when either one is provided. |
  | publicDirectoryCache | `boolean\|object` | `true` | Customize the `public`/`static` folder asset caching policy. Assigning an object with `value` and/or `test` lets you customize the caching policy and the types of files being cached. Assigning false disables caching |
  | useServerlessTraceTarget | `boolean` | `false` | Use the experimental-serverless-trace target to build your next app. This is the same build target that Vercel Now uses. See this [RFC](https://github.com/vercel/next.js/pull/8246) for details. |
  