Document S3 with_url path handling

[Herrtian]

Closes #409.

Clarifies that AmazonS3Builder::with_url does not retain the URL path, and points users to parse_url or the registry when they need the parsed path.

Checks:

• git diff --check

[Herrtian]

Looks like the Emulator Tests job failed while creating the LocalStack bucket before tests ran: Connection was closed before we received a valid response. The change here is docs-only and the other checks passed.