Document geoip6-db-digest in dir-spec.txt

There are extra-info descriptors containing "geoip6-db-digest" lines in the wild, but these are yet unspecified in dir-spec.txt. Not sure what other IPv6 stats related lines might be missing. This is related to legacy/trac#5053 (moved) and/or legacy/trac#5055 (moved).

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information