Document webpack change: Partial caching, expose webpack-sources

[webpack-bot]

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

See pull request: webpack/webpack#11345

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What kind of change does this PR introduce?
feature

Did you add tests for your changes?
partially, existing tests

Does this PR introduce a breaking change?
no

What needs to be documented once your changes are merged?
Plugins should consume require("webpack").sources instead of require("webpack-sources") to avoid version conflicts for persistent caching

[chenxsan]

I'll see where to document it.