Maint: (#18294) Update docs for ip property of Zone type (allows defrouters since 2.6)

Per Redmine 18294, the Zone type has allowed specifying default routers as part
of the IP string since at least 2.6 -- see Redmine 4296 for deets. This commit
updates the doc string to show how to do that.
Merge pull request #2134 from nfagerlund/docs_zone_defrouter

Maint: (#18294) Update docs for ip property of Zone type (allows defrouters since 2.6)