Handle self-ignore case with clear error (/_matrix/client/v3/user/{userId}/account_data/{type})

As the other side of https://github.com/matrix-org/synapse/issues/11963 I suggest to reject self-ignore on spec level.

Matrix shall reject a user ignoring themselves with `4xx` error.

The actual API call is
https://github.com/matrix-org/matrix-doc/blob/main/data/api/client-server/account-data.yaml

which is referenced from

https://github.com/matrix-org/matrix-doc/blob/main/content/client-server-api/modules/ignore_users.md
https://spec.matrix.org/v1.2/client-server-api/#put_matrixclientv3useruseridaccount_datatype

Start with data/api/client-server/account-data.yaml, then read content/client-server-api/modules/ignore_users.md and the referenced endpoint documentation. Define the documented behavior for a user attempting to ignore themselves, including the 4xx response, and update the specification so the rejection is explicit.

Written by the indexing model from the issue text.