docs: Document changing subdomains on multiple organizations page.
docs: Document changing subdomains on multiple organizations page.

andersk/zulip,andersk/zulip,andersk/zulip,zulip/zulip,andersk/zulip,zulip/zulip,andersk/zulip,zulip/zulip,rht/zulip,rht/zulip,rht/zulip,rht/zulip,rht/zulip,zulip/zulip,andersk/zulip,zulip/zulip,zulip/zulip,rht/zulip,zulip/zulip,rht/zulip,andersk/zulip