Document webpack change: Refactor getInitFragments

[webpack-bot]

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

See pull request: webpack/webpack#8388

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Refactor getInitFragments to a property in templateContext and apply method

Having only a single method in DependencyTemplate makes it easier to override

What kind of change does this PR introduce?
refactor

Did you add tests for your changes?
existing test

Does this PR introduce a breaking change?
yes, but with compat layer

What needs to be documented once your changes are merged?
DependencyTemplate.getInitFragments no longer exist and was replaced with a initFragments argument in the DependencyTemplate.apply method

[EugeneHlushko]

This is not in Docs, guess we keep this for migration guide