Issue title: Document webpack change: add more types and fix/remove TODOs
 Issue body: <!-- documentation request from webpack/webpack -->

*A pull request by @sokra was merged and maintainers requested a documentation change.*

See pull request: https://github.com/webpack/webpack/pull/9277

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**What kind of change does this PR introduce?**
refactoring
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**Did you add tests for your changes?**
existing test
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**Does this PR introduce a breaking change?**
yes
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**What needs to be documented once your changes are merged?**
* IgnorePlugins `checkContext` was removed
* Chunk.files is now a Set
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 Comments: 
 Comment 0: This is blocked tho until we update `next` cc @montogeek 
 Comment 1: Closing via #3149 
{'source': 'GithubIssues', 'repo_name_with_owner': 'webpack/webpack.js.org', 'issue_id': 456168675, 'repo_stars': 2275.0}