Document cases when auto_promtoe = false canary deploy, will auto promote

### Issue

There are a few cases when a canary deploy, with `auto_promote = false` will in fact automatically deploy.

The two primary cases are initial deploys, and a healthy canary deploy which occurs _after_ a previously failed deploy.

Start from the two cases described in the issue: initial deploys and a healthy canary deploy after a previously failed deploy. Review the relevant Nomad deployment documentation and explain when auto_promote = false still results in automatic promotion. Done means both cases and the expected behavior are clearly documented.

Written by the indexing model from the issue text.