doc: reassure novice they can change server host name
doc: reassure novice they can change server host name

It would be easy for a novice to create the first user before
fully realizing all the considerations for picking a server host name.
Reassure them by pointing to "upspin user -put".

Also, set the project with "gcloud config set project"
to avoid "--project" in the two gcloud commands
after "gcloud auth login" and perhaps save confusion
in future commands.

Change-Id: I65dc0fde65421850d84a57fcc9a14d5010a65f69
Reviewed-on: https://upspin-review.googlesource.com/7887
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@google.com>

upspin/upspin,upspin/upspin