Add ORM models / mixins documentation (#859)

[nbulaj]

• Adds YARD docs for models, concerns and mixins (ORM-specific code).
• Fix config docs:

[warn]: @param tag has unknown parameter name: Define
in file `lib/doorkeeper/config.rb' near line 102

[warn]: @param tag has unknown parameter name: ("Doorkeeper")
in file `lib/doorkeeper/config.rb' near line 114

[warn]: @param tag has unknown parameter name: the
in file `lib/doorkeeper/config.rb' near line 140

[warn]: @param tag has unknown parameter name: the
in file `lib/doorkeeper/config.rb' near line 151

• Addresses Improve API documentation #859

[houndci-bot]

Line is too long. [81/80]

[tute]

Thank you so much! 😃