Document webpack change: Expose the serialization API

[webpack-bot]

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

See pull request: webpack/webpack#8570

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What kind of change does this PR introduce?

feature

Did you add tests for your changes?

no

Does this PR introduce a breaking change?

no

What needs to be documented once your changes are merged?

webpack.util.serialization is now exposed.

[EugeneHlushko]

This is not in the docs, we should use this for migration guide mention

[ooflorent]

So we should stop opening PR for the public API? What’s the correct place?