Document webpack change: remove originModule from Dependencies and add parent module to ModuleGraph

[webpack-bot]

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

See pull request: webpack/webpack#7861

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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?
Some dependencies had originModule constructor argument and property. This is no longer the case. One can ask the ModuleGraph for the (origin) module via getParentModule.