upspin: document Put and PutLink more carefully
upspin: document Put and PutLink more carefully

The introduction to the Put documentation was difficult to understand.

For PutLink, there is new semantics.
The new name must not be an Access or Group file, and the old name
is not evaluated except to be cleaned.

Change-Id: Id069b1b822e02471b3fe916b1fca2220134b3131
Reviewed-on: https://upspin-review.googlesource.com/4261
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@google.com>
Reviewed-by: Eduardo Pinheiro <55040cbb39ba395339ae7940d8da6cd97797a4a8@google.com>

upspin/upspin,upspin/upspin