Document webpack change: Move HMR hotBoostrap hook

[webpack-bot]

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

See pull request: webpack/webpack#7816

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⚠️ Disable whitespace diff for better review experience!

What kind of change does this PR introduce?

refactoring

Did you add tests for your changes?

no

Does this PR introduce a breaking change?

yes

What needs to be documented once your changes are merged?

I don't if it is already documented but the API reference could outdated.
MainTemplate.hooks.hotBootstrap is replaced by HotModuleReplacementPlugin.getMainTemplateHooks(mainTemplate).hotBootstrap