Add option to exclude methods for Style/DocumentationMethod cop

[blackst0ne]

Is your feature request related to a problem? Please describe.

The Style/DocumentationMethod checks for all the public methods including initialize.
The constructor doesn't have to have a documentation comment.

Describe the solution you'd like

Add an option for the cop to exclude method names. E.g.

Style/DocumentationMethod:
  Enabled: true
  Include:
    - 'app/policies/**/*.rb'
  ExcludedMethods:
  - 'initialize'
  - 'foo'
  - 'bar'

Describe alternatives you've considered

An alternative is to disable the cop by the rubocop:disable comment around wanted methods.

[koic]

Looks reasonable to me. I've opened #11506.