Add comments to new CI config classes and modules

lib/gitlab/ci/config.rb

 module Gitlab
   module Ci
+    ##
+    # Base GitLab CI Configuration facade
+    #
     class Config
-      class LoaderError < StandardError; end
-
       delegate :valid?, :errors, to: :@global
       ##

lib/gitlab/ci/config/node/configurable.rb

@@ -2,6 +2,17 @@ module Gitlab
   module Ci
     class Config
       module Node
+        ##
+        # This mixin is responsible for adding DSL, which purpose is to
+        # simplifly process of adding child nodes.
+        #
+        # This can be used only if parent node is a configuration entry that
+        # holds a hash as a configuration value, for example:
+        #
+        # job:
+        #   script: ...
+        #   artifacts: ...
+        #
         module Configurable
           extend ActiveSupport::Concern

lib/gitlab/ci/config/node/entry.rb

@@ -2,6 +2,9 @@ module Gitlab
   module Ci
     class Config
       module Node
+        ##
+        # Base abstract class for each configuration entry node.
+        #
         class Entry
           class InvalidError < StandardError; end

lib/gitlab/ci/config/node/global.rb

@@ -2,6 +2,10 @@ module Gitlab
   module Ci
     class Config
       module Node
+        ##
+        # This class represents a global entry - root node for entire
+        # GitLab CI Configuration file.
+        #
         class Global < Entry
           include Configurable

lib/gitlab/ci/config/node/null.rb

 module Gitlab
   module Ci
     class Config
+      ##
+      # This class represents a configuration entry that is not being used
+      # in configuration file.
+      #
+      # This implements Null Object pattern.
+      #
       module Node
         class Null < Entry
           def value

lib/gitlab/ci/config/node/script.rb

@@ -2,6 +2,14 @@ module Gitlab
   module Ci
     class Config
       module Node
+        ##
+        # Entry that represents a script.
+        #
+        # Each element in the value array is a command that will be executed
+        # by GitLab Runner. Currently we concatenate this commands with
+        # new line character as a separator what is compatbile with
+        # implementation in Runner.
+        #
         class Script < Entry
           include ValidationHelpers