Add "SSH Configuration" section
Add "SSH Configuration" section to "borg serve" documentation, to outline ssh/sshd configuration to prevent borg serve keeping a lock on a repo in the event the ssh connection is abnormally disconnected.
In response to issues #3988, #636 and #4485 (and probably others).
drop BORG_HOSTNAME_IS_UNIQUE (please use BORG_HOST_ID if needed)
borg now always assumes it has a unique hostid - either automatically
from fqdn plus uuid.getnode() or overridden via BORG_HOST_ID.
