Add nodoc to CredentialsGenerator and MasterKeyGenerator [ci skip]

These classes are internally used only.

Author: y-yagi

railties/lib/rails/generators/rails/credentials/credentials_generator.rb

@@ -6,7 +6,7 @@
 
 module Rails
   module Generators
-    class CredentialsGenerator < Base
+    class CredentialsGenerator < Base # :nodoc:
       def add_credentials_file
         unless credentials.content_path.exist?
           template = credentials_template

railties/lib/rails/generators/rails/master_key/master_key_generator.rb

@@ -7,7 +7,7 @@
 
 module Rails
   module Generators
-    class MasterKeyGenerator < Base
+    class MasterKeyGenerator < Base # :nodoc:
       MASTER_KEY_PATH = Pathname.new("config/master.key")
 
       def add_master_key_file