Document webpack change: Performance fixes for next branch, watchpack update

[webpack-bot]

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

See pull request: webpack/webpack#8601

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updated watchpack. It now passes more info to webpack i. e. for missing files/directories.

What kind of change does this PR introduce?
refactoring, deps

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?

• Removed compilationParams.compilationDependencies
• Plugins can add dependencies to the compilation by adding to compilation.file/context/missingDependencies
• Changed hooks in NormalModuleFactory from waterfall to bailing, changed and renamed hooks that return waterfall functions
• Changed WatchFileSystem interface