remote-hostname-lookup: Clarify edge cases and error hanlding

This needs review from someone with knowledge of C Tor, I think.

This branch is on top of !301 (merged) which I hope isn't controversial.

assigned to @Diziet

changed the description

mentioned in issue tor#40248 (closed)

requested review from @ahf

FTR, I don't think this is at all urgent, but it's relevant if someone wants to try to fix tor#40248 (closed)

(wrong ticket comment deleted)