Document webpack change: allow multiple entryModules per chunk

[webpack-bot]

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

See pull request: webpack/webpack#7896

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• MultiModule was removed
• webpack can now handle multiple entrypoints natively and don't need to create a MultiModule for this
• move entry modules into ChunkGraph
• allow to specify ChunkGroup to depend on for entry modules

What kind of change does this PR introduce?
refactor

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?

• Chunk.entryModule no longer exist -> ChunkGraph