Document best practice for moving migrations from EE to CE

The following discussion from !10437 (merged) should be addressed:

• @ifarkas started a discussion: (+7 comments)

  @abrandl, I tried to implement your suggestion in

  First migration to add all new columns to application_settings (with transactions enabled, add_column, no column_exists? necessary)

  with:

  class MoveExternalClassificationSettingsInApplicationSettingsToCore < ActiveRecord::Migration[5.0]
    DOWNTIME = false
  
    def change
      change_table :application_settings do |t|
        t.boolean :external_authorization_service_enabled, default: false
        t.string :external_authorization_service_url
        t.string :external_authorization_service_default_label
        t.float :external_authorization_service_timeout, default: 0.5
        t.text :external_auth_client_cert
        t.text :encrypted_external_auth_client_key
        t.string :encrypted_external_auth_client_key_iv
        t.string :encrypted_external_auth_client_key_pass
        t.string :encrypted_external_auth_client_key_pass_iv
      end
    end
  end

  But then I am getting

  ActiveRecord::StatementInvalid: PG::DuplicateColumn: ERROR:  column "external_authorization_service_enabled" of relation "application_settings" already exists

  So it seems those column_exists? checks are necessary anyway. Or am I missing something?