Specify the process by which historical synapse behaviour (from before v1, 2019) is written into the spec after the fact

Context: [in #spec](https://matrix.to/#/!NasysSDfxKxZBzJJoE:matrix.org/$Yaacju56xdPQdkXlKzEE9faGizRRKLKFBhVv8Wrl2tU?via=matrix.org&via=libera.chat&via=envs.net) an issue came up regarding Synapse's history with the spec, and that a process exists (which has not been documented or alluded to) which judges if diversions from the spec (by synapse) should be incorporated in the spec as canonical, or rejected.

> For clarity: people should be using the spec, not synapse, as their reference. When the spec fails them, they should open bugs on the matrix-doc repo so we can triage them, figure out what is going on, and make a plan to fix it.
>
> Sometimes this will mean documenting something that wasn't there before (because Matrix is complicated and hard to document all at once), and sometimes it will mean saying "sorry, your implementation did something illegal"

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I'd request that there's an appendix (similar in nature to the spec process) that details the way these issues are dealt with, judging a "synapse bug" either as canonical to the spec, or as divergent.