Document event size errors (SPEC-406)

The "Event structure" subsection "Size limits" in the C-S API spec states some data size limits for events and for some of the events' fields. There should be an explanation of what a homeserver is expected to do if it receives an event which violates one of these limits, as well as how clients and servers should behave when wanting to send an event that would violate the limits, e.g. an event with an extremely long message body.

(Imported from https://matrix.org/jira/browse/SPEC-406)

(Reported by Jimmy Cuadra)

Open the C-S API spec and find the “Event structure” → “Size limits” subsection. Document what homeservers should do when receiving over-limit events and how clients and servers should handle sending them, including the long-message example; done when the behavior for each stated limit is explicit.

Written by the indexing model from the issue text.