Document webpack change: add output.charset option

[webpack-bot]

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

See pull request: webpack/webpack#11333

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What kind of change does this PR introduce?

feature. closes webpack/webpack#9327

Did you add tests for your changes?

Yes

Does this PR introduce a breaking change?

No

What needs to be documented once your changes are merged?

Setting output.charset to false will stop webpack from adding charset="utf-8" to <script> tag.