api docs: Document that new messages might be already ready.
api docs: Document that new messages might be already ready.

This is an important and somewhat subtle detail that we'd like to help
clients implement correctly.

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