Add note about action names versus ActionController reserved methods

In both Rails 5.2 and Rails 6.1, defining a controller action method
named `config` will result in a `SystemStackError: stack level too deep`
exception for all requests routed to that controller. This is because
`ActiveSupport` defines `ActiveSupport::Configurable#config` which is
included into `ActionController::Base` by default, and the new config
action overrides it. Any actions in the controller will call `render`
which eventually will call `logger` which is a configurable attribute
which calls `config` which then calls the new `config` action which
calls `render` and so on. `config` is not the only method that will
trigger this behavior if redefined in a controller: In Rails 6.1, there
are 17 methods that would result in `SystemStackError` if redefined, 9
that would result in `ArgumentError`, and 3 that would result in a
`AbstractController::DoubleRenderError`. Most of these methods are
obvious that they should be avoided, like `render`, but some, including
`config` since it's never something the user would typically call
themselves and its buried deep down in some dependencies, are surprising
when encountered and the `SystemStackError` that simply points back to
the action method isn't very helpful when trying to debug what has
happened.

This commit updates the ActionController Overview section of the Guide
to add a note about the potential for this conflict, but stops short of
a full list of reserved methods since it's a.) lengthy, and b.) likely
to change as internal APIs are updated.

Closes to rails#41323

Author: Chris Bloom

guides/source/action_controller_overview.md

@@ -65,7 +65,12 @@ The [Layouts and Rendering Guide](layouts_and_rendering.html) explains this in m
 
 Only public methods are callable as actions. It is a best practice to lower the visibility of methods (with `private` or `protected`) which are not intended to be actions, like auxiliary methods or filters.
 
+WARNING: Some method names are reserved by Action Controller. Accidentally redefining them as actions, or even as auxiliary methods, could result in `SystemStackError`. If you limit your controllers to only RESTful [Resource Routing][] actions you should not need to worry about this.
+
+NOTE: If you must use a reserved method as an action name, one workaround is to use a custom route to map the reserved method name to your non-reserved action method.
+
 [`ActionController::Base`]: https://api.rubyonrails.org/classes/ActionController/Base.html
+[Resource Routing]: https://guides.rubyonrails.org/routing.html#resource-routing-the-rails-default
 
 Parameters
 ----------