Document webpack change: rename index/index2 to pre/postOrderIndex

[webpack-bot]

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

See pull request: webpack/webpack#7986

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What kind of change does this PR introduce?
refactor

Did you add tests for your changes?
existing tests

Does this PR introduce a breaking change?
no (but it deprecate something)

What needs to be documented once your changes are merged?

• set/getModuleIndex -> set/getModulePreOrderIndex
• set/getModuleIndex2 -> set/getModulePostOrderIndex