Document the fact that StatFs.restat() and ctor can throw.

StatFs.restat() and the StatFs constructor can throw
IllegalArgumentException. This was not previously documented;
not all callers took this into account, for example:
   http://r.android.com/251290

This CL adds documentation to those methods. It also adds
comments to two of the callers.

Separately from this CL, we may in addition consider adding
new API StatFs.checkedRestat() and StatFs.checkedCreate()
or similar that throw IOException; we cannot change the
existing constructor and method since they are public.

Test: Checked that "make" still completed successfully.

Change-Id: I6a0b3cb7718939408937c61de7c3b000b948fa59