Document which address functions we should use, and when

nickm says that there are a lot of ways to get an address in Tor.

Let's document the functions that are left at the end of legacy/trac#23975 (moved), and when each of them should be used. It would be nice to have a summary, and then also details in each function comment.

We'll need to include these functions:

• node_get_*_*port()
• *_choose_address()
• any others?