docs: Document TODO/compatability convention in release checklist.
docs: Document TODO/compatability convention in release checklist.

This should provide a better mechanism for us to keep track of what
backwards-compatibility code we can safely delete.

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