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 https://github.com/rails/rails/issues/41323