docs, add missing `:nodoc: for `Associations::Builder`. [ci skip]

This class is only used internally. We should keep it out of public
documentation. This patch adds nodoc for
`ActiveRecord::Associations::Builder` and everything nested within.

activerecord/lib/active_record/associations/builder/association.rb

@@ -9,7 +9,7 @@
 #    - CollectionAssociation
 #      - HasManyAssociation
 
-module ActiveRecord::Associations::Builder
+module ActiveRecord::Associations::Builder # :nodoc:
   class Association #:nodoc:
     class << self
       attr_accessor :extensions

activerecord/lib/active_record/associations/builder/belongs_to.rb

-module ActiveRecord::Associations::Builder
+module ActiveRecord::Associations::Builder # :nodoc:
   class BelongsTo < SingularAssociation #:nodoc:
     def self.macro
       :belongs_to

activerecord/lib/active_record/associations/builder/collection_association.rb

@@ -2,7 +2,7 @@
 
 require 'active_record/associations'
 
-module ActiveRecord::Associations::Builder
+module ActiveRecord::Associations::Builder # :nodoc:
   class CollectionAssociation < Association #:nodoc:
 
     CALLBACKS = [:before_add, :after_add, :before_remove, :after_remove]

activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb

-module ActiveRecord::Associations::Builder
+module ActiveRecord::Associations::Builder # :nodoc:
   class HasAndBelongsToMany # :nodoc:
-    class JoinTableResolver
+    class JoinTableResolver # :nodoc:
       KnownTable = Struct.new :join_table
 
-      class KnownClass
+      class KnownClass # :nodoc:
         def initialize(lhs_class, rhs_class_name)
           @lhs_class      = lhs_class
           @rhs_class_name = rhs_class_name

activerecord/lib/active_record/associations/builder/has_many.rb

-module ActiveRecord::Associations::Builder
+module ActiveRecord::Associations::Builder # :nodoc:
   class HasMany < CollectionAssociation #:nodoc:
     def self.macro
       :has_many

activerecord/lib/active_record/associations/builder/has_one.rb

-module ActiveRecord::Associations::Builder
+module ActiveRecord::Associations::Builder # :nodoc:
   class HasOne < SingularAssociation #:nodoc:
     def self.macro
       :has_one

activerecord/lib/active_record/associations/builder/singular_association.rb

 # This class is inherited by the has_one and belongs_to association classes
 
-module ActiveRecord::Associations::Builder
+module ActiveRecord::Associations::Builder # :nodoc:
   class SingularAssociation < Association #:nodoc:
     def self.valid_options(options)
       super + [:dependent, :primary_key, :inverse_of, :required]