Update db_import help with authoritative formats

Taken from import_filetype_detect in lib/msf/core/db.rb.

[SeeRM #8799]
Update core/db.rb comments 'n' stuff
Update CommandDispatcher::Db spec
Add missing formats in lib/msf/core/db.rb comment

Found outside big if block. Ugh.
Land #3341, db_import help update, more formats