Document webpack change: avoid collision in `data-webpack` by adding the uniqueName

[webpack-bot]

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

See pull request: webpack/webpack#11065

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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?

• data-webpack has output.uniqueName + : as prefix

[jeffin143]

On it

dropping it off, since I am not sure what to document , open to take