documentation about state key unclear or accidentally removed

[julianhille]

Describe the bug
There was a documentation about the state key in the client server spec, which stated:

| Key          | Type         | Description                                                                                                  |
|--------------|--------------|--------------------------------------------------------------------------------------------------------------|
| state_key    | string       | **Required.** A unique key which defines the overwriting semantics for this piece of room state. This value is often a zero-length string. The presence of this key makes this event a State Event. State keys starting with an `@` are reserved for referencing user IDs, such as room members. With the exception of a few events, state events set with a given user's ID as the state key MUST only be set by that user.         |

The part about the user ID and the zero length is missing now and has been removed in this PR by @richvdh :
matrix-org/matrix-spec-proposals#3658 from this file https://github.com/matrix-org/matrix-spec-proposals/blob/5b44bc4bd71baa5ffbd71b4fae921035aa291bd1/content/client-server-api/_index.md

Too me this looks like an accidental remove and it is not clear to me if the code here in synapse: https://github.com/matrix-org/synapse/blob/develop/synapse/event_auth.py#L549

is outdated / wrong or if it is correct and the documentation is not specific enough anymore.

[richvdh]

oops, matrix-org/matrix-spec-proposals#3658 was not intended to change the spec here.