Document webpack change: DefinePlugin: add ability to use function return values

[webpack-bot]

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

See pull request: webpack/webpack#6793

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What kind of change does this PR introduce?

This adds the following API to DefinePlugin:

new webpack.DefinePlugin({
  BUILT_AT: webpack.DefinePlugin.runtimeValue(Date.now, [fileDep, fileDep])
})

Did you add tests for your changes?

Yes

If relevant, link to documentation update:

Not yet

Summary

I wanted to inject an RPC schema into my builds, and when that schema changes, I want webpack-dev-server to rebuild and pick up the updated schema.

See #6749

Does this PR introduce a breaking change?

No

Other information

This PR is not ready to be merged (see comments inside), but I was hoping for some initial feedback.

[danielthank]

I would like to proceed this.

[sokra]

Passing true as second argument instead of fileDependencies will flag the module as uncachable.

[EugeneHlushko]

I am closing it but the mention of true instead of array is very cryptic, @sokra maybe you can help improving it? To me its not clear, what means will flag the module as uncachable., which module? define plugin is used in the webpack config, so does it mean it flags the config itself?

Here's the link to the doc we added:

https://webpack.js.org/plugins/define-plugin/#runtime-values-via-runtimevalue