docs(cli): add mention of our default port behavior (#62964)
## Changes
- We need to document how that we retry until a port is found when we're
not specifying a port via the CLI (`--port`) or the `PORT` environment
variable. For example, if `3000` and `3001` are used, we will check
`3000`, `3001`, and then land on `3002`.
Closes NEXT-2731
