Add documentation for add_flash_types [ci skip]

Document a little bit the ActionController::Flash::add_flash_types
class method.
Merge pull request #11394 from robin850/patch-8

Add documentation for add_flash_types [ci skip]
 #11381: Ignore config.eager_load=true for rake

Closes #11381
Merge pull request #11389 from jetthoughts/11381_fix_hit_database_on_precompile

 #11381: Ignore config.eager_load=true for rake