doc: clarify that du operands are interdependent

doc: clarify that du operands are interdependent

Following on from http://bugs.gnu.org/17546
make it more obvious that du may elide specified operands
to avoid double counting in the set.

* src/du.c (usage): Specify that du operates on the set of
operands, rather than each independently.
* doc/coreutils.texi (du invocation): Likewise.  Also state
that the number of entries printed may change due to the
order specified.  Currently, deeper items specified earlier
will result in them being displayed, but don't mention that
implementation detail in the documentation.
* THANKS.in: Add reporter.
Reported by Stephen Shirley