Document webpack change: fix ExportsInfo

[webpack-bot]

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

See pull request: webpack/webpack#15367

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• should mark every export canMangleProvide=false if there are unknown exports
• add __webpack_exports_info__.<name>.canMangle to api
• add test case

What kind of change does this PR introduce?

bugfix
fixes #15214

Did you add tests for your changes?

yes

Does this PR introduce a breaking change?

no

What needs to be documented once your changes are merged?

add __webpack_exports_info__.<name>.canMangle to api