upspin: document that file has no holes
upspin: document that file has no holes

As discussed, we do not allow sparse files.

Change-Id: Iec6125b3e4cd95874ac3b9a378c9cfa320c5398f
Reviewed-on: https://upspin-review.googlesource.com/2791
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@google.com>

upspin/upspin,upspin/upspin