add documentation and example for mail_relay_domain_delete function #6399

Amavisd supports wildcards by performing decreasingly specific SQL
lookups:
    9 - lookup for user+foo@sub.example.com
    8 - lookup for user@sub.example.com
    7 - lookup for user+foo
    6 - lookup for user
    5 - lookup for @sub.example.com
    3 - lookup for @.sub.example.com
    2 - lookup for @.example.com
    1 - lookup for @.com
    0 - lookup for @.       (catchall)
    (https://www.ijs.si/software/amavisd/README.lookups.txt)

However idn_to_* returns an empty string if the domain has a leading dot
which means lookups 0-3 cannot be used.  This is fixed by removing the
leading dot before encoding or decoding and adding it back just before
returning the domain.