upspin: fix documentation
upspin: fix documentation

Dave was confused by the exact behavior and this
should clarify it.

Change-Id: Ic8ccdd7fd8e6427e58a1f66e8361ceae7e2adf97
Reviewed-on: https://upspin-review.googlesource.com/6642
Reviewed-by: Eric Grosse <f6ae9ca1078fda92fd7c2c20411c29bc6711159e@google.com>
Reviewed-by: David Presotto <516b9783fca517eecbd1d064da2d165310b19759@google.com>

upspin/upspin,upspin/upspin