a setting for disabling documentation to internal APIs

[dnesteryuk]

Our application has 3 APIs, 2 of them are internal and don't require documentation. However, there has not been any option to prevent Grape from documenting parameters for internal APIs.

This change adds a do_not_document! setting which instructs Grape to not document parameters, thus needless objects allocation is avoided.

class Api < Grape::API
  do_not_document!
end

The logic for documenting parameters was moved to a separate class, the Grape::Validations::ParamsScope class has to many responsibilities, it would be better to split it.

Our API has lots of endpoints like this:

resource :addresses do
  params do
    requires :address, type: Hash do
    requires :street, type: String
    requires :postal_code, type: Integer
    optional :city, type: String
  end
  post '/' do
    declared(params, include_missing: false)
    'Done'
  end
end

With documentation:

Total allocated: 342247 bytes (3186 objects)
Total retained:  70915 bytes (594 objects)

Without documentation:

Total allocated: 341183 bytes (3178 objects)
Total retained:  69091 bytes (580 objects)

1,78 Kb is saved.

[dblock]

I'm good with this, @dmitry care to CR too?

[dm1try]

LGTM
though, some of those changes are refactoring not related to documentation, I would prefer to have separate commits for that next time.