api docs: Clarify limits on use of propagate_mode.
api docs: Clarify limits on use of propagate_mode.

hackerkid/zulip,kou/zulip,kou/zulip,zulip/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,andersk/zulip,andersk/zulip,punchagan/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,andersk/zulip,rht/zulip,hackerkid/zulip,andersk/zulip,rht/zulip,zulip/zulip,kou/zulip,hackerkid/zulip,eeshangarg/zulip,hackerkid/zulip,rht/zulip,zulip/zulip,rht/zulip,hackerkid/zulip,punchagan/zulip,andersk/zulip,eeshangarg/zulip,eeshangarg/zulip,kou/zulip,punchagan/zulip,punchagan/zulip,punchagan/zulip,rht/zulip,hackerkid/zulip,zulip/zulip,andersk/zulip,kou/zulip,punchagan/zulip,punchagan/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,eeshangarg/zulip,hackerkid/zulip