Document webpack change: feat: allow to customize the stage of BannerPlugin

[webpack-bot]

A pull request by @alexander-akait was merged and maintainers requested a documentation change.

See pull request: webpack/webpack#18204

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What kind of change does this PR introduce?

feat, fixes webpack-contrib/css-minimizer-webpack-plugin#64

Did you add tests for your changes?

WIP

Does this PR introduce a breaking change?

No

What needs to be documented once your changes are merged?

Yes, need to document the stage option

usage:

new webpack.BannerPlugin({
  raw: true,
  banner: "/* banner is a string */",
  stage: webpack.Compilation.PROCESS_ASSETS_STAGE_REPORT
})

It will add the banner message after minimizers and any assets manipulation