doc: clarify usage of suffixed users
doc: clarify usage of suffixed users

Updates #215

Change-Id: I340b610b02eb97213246515a0ee290b90f3cd7ec
Reviewed-on: https://upspin-review.googlesource.com/8210
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin