Clarify which events come down on which keys in /sync (SPEC-297)

Submitted by @​kegan:matrix.org
At the moment things like the ephemeral block have an unhelpful description just saying that typing is something that can appear in that block. We should explicitly state which events go where or provide a way to deduce them based on the kind of event and then explain the steps you perform to work it out

(Imported from https://matrix.org/jira/browse/SPEC-297)

Start with the /sync section of the Matrix protocol specification and review the description of the ephemeral block. Document which event types appear on each key, or explain how to derive their placement from the event kind; done means the mapping or deduction steps are explicit.

Written by the indexing model from the issue text.