docs: Clarify possible call status values

[bminer]

For 'connected' and 'ended' Call events, what are the possible call status values?

I understand that Twilio provides a list of possible call statuses, but I cannot figure out which of those apply to which event.

An official reply from a Twilio representative would be appropriate here or a reference URL to Twilio documentation that clarifies this issue.

[bminer]

This has been clarified a bit in the documentation, but I still have to talk with someone from Twilio to get an official response. For now, users wondering about this should trust my gut instinct and stick to the values listed in the node-twilio-api docs (i.e. README.md). Thanks.