Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.
Add documentation comments for volume expand controller
These comments help clear out some of the design choices made
in code.
cc @kubernetes/sig-storage-pr-reviews @NickrenREN