doc: note update of dovecot2 and related packages

mail/dovecot2 2.3.7.2
mail/dovecot2-gssapi 2.3.7.2
mail/dovecot2-ldap 2.3.7.2
mail/dovecot2-mysql 2.3.7.2
mail/dovecot2-pgsql 2.3.7.2
mail/dovecot2-sqlite 2.3.7.2
mail/dovecot2-pigeonhole 0.5.7.2

(taca)