models: Improve comment documentation around Recipients and Huddles.
models: Improve comment documentation around Recipients and Huddles.

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