Document webpack change: Move Module.used and Module.usedExports into ModuleGraph

[webpack-bot]

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

See pull request: webpack/webpack#7852

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What kind of change does this PR introduce?
refactor

Did you add tests for your changes?
existing tests

Does this PR introduce a breaking change?
yes

What needs to be documented once your changes are merged?
Module.used and Module.usedExports no longer exist and Module.getUsedExports should be used instead.

[Lalitha-Iyer]

Couldn't find this in the docs