Fix lazy load hooks documentation.

[kaspth]

Active Support has long had load hooks in https://github.com/rails/rails/blob/39b8b8fdbf6c175a64f07198ff1c950e33c72cd0/activesupport/lib/active_support/lazy_load_hooks.rb, but since 7c90d91 the docs stopped showing up on http://api.rubyonrails.org/files/activesupport/lib/active_support/lazy_load_hooks_rb.html.

Your mission, should you choose to accept it, is to fix this. We could wrap the file's behavior in a module and extend Active Support with it inline.

Then for good measure we should give the docs some housekeeping. Try coming up with a better explanation for why the lazy load hooks exist. Use this as a starting ground: rails/rails-controller-testing#24 (comment)

[mrageh]

@kaspth I'll pick this up

[kaspth]

Sweet!

[vipulnsward]

@kaspth I believe this can be closed?

[mrageh]

@vipulnsward I'd like to add another commit that improves the docs around load hooks

[kaspth]

Yeah, this still encompasses a doc upgrade. So let's keep it open for now.

@mrageh you can add "Fixes #25784" in your PR and then this issue will auto-close when we merge that. Less work for us, thanks :)