Document external ID

[iloveitaly]

It's unclear exactly what the external ID does. Is it just transaction metadata? It seems to protect against duplicates. However, if you delete transaction(s) in LM it's not clear if you can add another transaction with the same external ID again.

[dylanmazurek]

I'm running into this issue, I am syncing between systems and getting duplicate external_id errors but when searching for the transaction via the API, none exist.