more details in xchammer Diags.bzl tool comment

https://github.com/pinterest/xchammer/blob/d39ca528ae9147f86c054db1a1ee0b52b05bb5dc/tools/XCConfigExporter/main.swift#L59

If that comment had more details of how to invoke the command (perhaps to a link on this site with details) it would help others learn more about how to update these items. Thanks!