Document the files in $datadir/keys, maybe in the man page?

Nick and I were just pondering the legacy v3 signing key feature. I told him to go to the man page and look for the word 'legacy'. Then I actually did it: I found "V3AuthUseLegacyKey", but no further instructions on how to use it.

I assume the next step is for people who want to use it to go to src/or/config.c and track down other words that include the substring "egacy".

Is there some better place to look? Should the man page list at least the possible filenames in $datadir/keys/ ?