Document webpack change: move Module.issuer into Module.getIssuer(moduleGraph)

[webpack-bot]

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

See pull request: webpack/webpack#7870

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What kind of change does this PR introduce?
refactor

Did you add tests for your changes?
N/A

Does this PR introduce a breaking change?
yes

What needs to be documented once your changes are merged?
Module.issuer is now Module.getIssuer(ModuleGraph)