access: improve documentation for Access.Can
access: improve documentation for Access.Can

There are important cases that it handles that the caller need not
worry about, but they were not documented:
  - owner has read and list.
  - only owner can update Group and Access files.

Documentation change only.

Change-Id: Ia7a75a86f2f9e5dbf5568d6021202d147c1b753e
Reviewed-on: https://upspin-review.googlesource.com/2651
Reviewed-by: Eduardo Pinheiro <55040cbb39ba395339ae7940d8da6cd97797a4a8@google.com>
Reviewed-by: Eric Grosse <f6ae9ca1078fda92fd7c2c20411c29bc6711159e@google.com>
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@google.com>

upspin/upspin,upspin/upspin