[SPARK-23112][DOC] Update ML migration guide with breaking and behavior changes.#20421

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MLnick wants to merge 6 commits intoapache:masterfrom

MLnick:SPARK-23112-ml-guide

Commits on Jan 23, 2018

Add highlights and migration guide for 2.3
Nick Pentreath
committed

Commits on Jan 29, 2018

Merge branch 'master' into SPARK-23112-ml-guide
Nick Pentreath
committed
Update migration guide with breaking changes
Nick Pentreath
committed

Commits on Jan 30, 2018

Update behavior change items
Nick Pentreath
committed
Add more behavior change items
Nick Pentreath
committed
Update MLOR summary note
Nick Pentreath
committed