Document webpack change: rename optimization.splitChunks.cacheGroups.vendors -> defaultVendors

[webpack-bot]

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

See pull request: webpack/webpack#7637

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• rename optimization.splitChunks.cacheGroups.vendors -> defaultVendors
• add automaticNamePrefix option
• allow automaticNameDelimiter on cacheGroup level (it is already that way in the doc)

What kind of change does this PR introduce?
rename, feature

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?
see above