Zygote: clean up comment for isolateAppData()

This comment has several inaccuracies, such as saying that tmpfs is
mounted on /mnt/expand (it's actually /mnt/expand/$volume/user{,_de}),
and referring to DE storage when CE storage is meant.  Fix these
inaccuracies, and rework it to explain more clearly what's going on.

Change-Id: I97106f1c2551906c12a96f17934db9253a7fe605