docs(hooks.md, model.js): add warnings about memory limitations of individualHooks

[jedwards1211]

Pull Request check-list

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Not applicable, this is a document only change

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Description of change

I was very disappointed to realize today that if were to update or destroy a billion instances with individual hooks, Sequelize would try to fetch all billion of them into memory before running the individual hooks. I had mistakenly assumed it would iterate through them in batches or using cursors of some kind...

I wish I had been warned about this before I decided to start using hooks, so I'm adding this warning to the docs.

[codecov]

Codecov Report

Merging #9881 into master will decrease coverage by 4.7%.
The diff coverage is n/a.

[jedwards1211]

Codecov report that coverage will decrease is totally bogus, I haven't changed any code at all. Are there race conditions in the tests?

[sushantdhiman]

Remove all changes from this file, it is ok to have this as a documentation only warning.

[jedwards1211]

@sushantdhiman what do you think is the downside to also warning in this file?
The Sequelize documentation is large and a warning in the tutorial could be easily missed (IIRC I learned about individualHooks in the reference docs before seeing it in the tutorial docs.) Having this warning in more places makes it more likely that devs will see it.

[sushantdhiman]

API docs should not cover such warnings or expanded explanation, they should only explain working of any give option and platform limitation if any

[jedwards1211]

Okay, would it be okay for the API docs to simply say "all instances will be fetched into memory before running hooks"? That is explaining what it does...

[sushantdhiman]

SELECT all records matching the where parameter... it should be clear by that. SELECTED records will be loaded to memory and large SELECT surface will consume more memory

[jedwards1211]

Alright, I removed the changes to model.js