All APIs should have a dedicated "Unsupported Endpoints" section

[turt2live]

Link to problem area:

• https://spec.matrix.org/v1.10/client-server-api/#common-error-codes (M_UNRECOGNIZED)
• https://spec.matrix.org/v1.10/identity-service-api/#standard-error-response (M_UNRECOGNIZED)

Issue

A common thing I do is try to link to the 404/405 M_UNRECOGNIZED behaviour, but because it's buried in the "Common error codes", I need to include language to navigate the reader. Preferably, a section exists which can be directly linked to, like the following:

• https://spec.matrix.org/v1.10/server-server-api/#unsupported-endpoints
• https://spec.matrix.org/v1.10/application-service-api/#unknown-routes
• https://spec.matrix.org/v1.10/push-gateway-api/#unsupported-endpoints

[turt2live]

element-hq/synapse#17153 is an example of that awkward navigation language:

[...] as per the M_UNRECOGNIZED description at https://spec.matrix.org/v1.10/client-server-api/#common-error-codes