Update how to configure Action Mailer interceptors/observers

[pixeltrix]

Unsure about the style of the example here - typically we use single lines like this:

Rails.application.config.action_mailer.interceptors = %w[MyInterceptor]

However it causes the configuration to wrap in the guides template so I re-wrote it to use the block form like this:

Rails.application.configure do
  config.action_mailer.interceptors = %w[MyInterceptor]
end

Is this pattern something we want to expose? It just uses instance_eval on the block.

[zzak]

I prefer the block syntax because Rails should already generate this for you, I'm curious why there wouldn't be a "staging.env" but that is not relevant here imo