Document webpack change: allow to disable wasm import mangle

[webpack-bot]

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

See pull request: webpack/webpack#7487

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disable it by default (temporary)

What kind of change does this PR introduce?
bugfix

Did you add tests for your changes?
existing test

Does this PR introduce a breaking change?
no

What needs to be documented once your changes are merged?
the option optimization.mangleWasmImports