Ignore rake files for coverage

MSP-9816
Add missing YARD docs

MSP-9816
Metasploit::ERD::Entity::Class

MSP-9816

Metasploit::ERD::Entity::Class find all directly related classes through
belongs_to associations on the class.  In addition to
Metasploit::ERD::Entity::Class, this includes
Metasploit::ERD::Entity::Relationship, which handles finding the related
classes for each belongs_to association and the differences between
polymorphic and non-polymorphic associations.