Clarifications to /sync description (#3534)
* Clarify that E0 and E1 are not returned
* Clarify that ASCII art diagram 2 refers to diagram 1
* Correct third ASCII art diagram
To match the spacing in. Borked in the move to new docs format.
https://github.com/matrix-org/matrix-doc/blob/55aed1d296bc9c2e9db558c23f6dd1ffade1abb3/specification/client_server_api.rst#L1610-L1615
* Make ASCII art 3 follow from ASCII art 2
* Explain how to fill the gap
* Create 3543.clarification
* Update content/client-server-api/_index.md
Co-authored-by: Travis Ralston <travisr@matrix.org>
