Document the format for S3_UPLOADS_BUCKET_URL

By default, S3 Uploads will use the canonical S3 URIs for referencing the uploads, i.e. `[bucket name].s3.amazonaws.com/uploads/[file path]`. If you want to use another URL to serve the images from (for instance, if you [wish to use S3 as an origin for CloudFlare](https://support.cloudflare.com/hc/en-us/articles/200168926-How-do-I-use-CloudFlare-with-Amazon-s-S3-Service-)), you should define `S3_UPLOADS_BUCKET_URL` in your `wp-config.php`:

```PHP

define( 'S3_UPLOADS_BUCKET_URL`, '[Your origin domain/URL]' );

// Define the base bucket URL (without trailing slash)

define( 'S3_UPLOADS_BUCKET_URL`, 'https://your.origin.url.example/path' );

```

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