Adding simple docs for ActionDispatch::Flash::FlashHash#to_session_value
adding nodoc to private methods
Use web_console 2.0 for 4.2.0.rc1 release

This one replaces the notable web-console mentions in guide and the
default Gemfile.
Merge pull request #17728 from jcutrell/jcutrell/docs-to_session_value-for-pr

Adding simple docs for ActionDispatch::Flash::FlashHash#to_session_value
Merge pull request #17773 from gsamokovarov/mention-web-console-2.0-rc1

Use web_console 2.0 for 4.2.0.rc1 release
Use jquery-rails 4.0.0
Clarify where result goes, and supported formats of generate task.

[ci skip]