Clarify autoload_paths and eager_load in guides per current usage.

[timdiggins]

Update the guide on autoloading (which is out of date with current practice and code) to include autoload_paths, eager_load_paths and eager_load in production.

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[timdiggins]

I edited the PR to be a lot more succinct and clear. The reasoning (which I removed) I put here:

---

I agree with #27268 (which this addresses) that the guide on autoloading is a bit out of date with regard to autoload_paths and eager_load.

I've run into this issue before (typically in upgrade / cross-compatibility situations) but not regularly enough to keep it in mind. Each time I've had to piece together how autoloading works. I've pieced it together (again) from:

• issues like Rails falls back on non-thread-safe autoloading even when eager_load is true #13142
• stack overflows https://stackoverflow.com/questions/38198668/rails-5-load-lib-files-in-production
• blog posts like https://collectiveidea.com/blog/archives/2016/07/22/solutions-to-potential-upgrade-problems-in-rails-5
• and this one
• and other issues like rails + rspec + spring + eager_load = uninitialized constant spring#519 (comment)) -- though this looks like a workaround for a workaround rather than a fix.
• rails source

So I've tried to document for others and my future self how things actually work with recommendations.

[kirs]

Should "rails" be in capital here?

[timdiggins]

Yes good catch I think you're right -- Rails is always capitalized in the guides (except for filenames / command lines obvs). I didn't spot that before. I'll make a new PR to address that (and any other comments)