This is a pass over the documentation which fills the missing gaps of
`ApplicationRecord`.

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Fixed a typo

config.autoload_paths is not changeable from environment specific configuration files.

* Fix a few typos
* Wrap some lines around 80 chars
* Rephrase some statements

Not sure how this one slipped, maybe deleted by accident while editing or
something. Anyway, fixed.

New wording is based on the "cref" sentence earlier in the section.

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Move a introduction of `Module.nesting` to the beginning of sentence.
This change will help reader to try to dump `nesting`.

- Changed `IN` to `ON` in all note sentences in guides.

References #18148.

The term 'constant autoloading' can be mistakenly
construed as 'constantly autoloading'.

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/cc @matthewd

The previous example was a little convoluted and the exposition
claifying the parts that were correct albeit not totally obvious
were interferring in my view.

This example has less things going on and gets to the key problem
with less balls in the air.

"Lookup" is a noun, but not a verb. The verb is "look up".

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I think "What" is was fine, "The purpose of" assumes the reader knows such thing exists.
"What is" is fine, and also matches the following "What is require_dependency".

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@fxn Great job with the guide, I read it all! Yeah!

Some paragraphs might be restructured, but I'll leave that to you
since you are currently working on it. This commit only includes
some minor typos and commas that in my opinion clarify the reading.

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Follow-up to previous commit

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Fix previous merge

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