Document webpack change: disable splitChunks.minRemainingSize for multiple selected chunks

[webpack-bot]

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

See pull request: webpack/webpack#9257

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What kind of change does this PR introduce?
bugfix

Did you add tests for your changes?
yes

Does this PR introduce a breaking change?
yes

What needs to be documented once your changes are merged?
minRemainingSize is only active when a single chunk remains

[EugeneHlushko]

via #3174