netdir: Conditionally expose APIs for working with unusable relays; document what "usable" means.

This MR is based on top of !1310 (merged); only the last 3 commits here are new. Please don't merge this until !1310 (merged) is merged.

There are two main changes here based on a conversation with @juga about Margot's needs:

• NetDir now exposes two functions, gated on experimental-api, to find out whether a relay is present in the consensus. (Existing functions for this purpose can't tell you about the case where the relay is in the consensus but has no MD yet.)
• We now do a better job of documenting what a "usable" relay is.