Federation: wrong endpoint for device list resync

[The federation spec says](https://spec.matrix.org/v1.6/server-server-api/#device-management):

> If a server receives an EDU which refers to a `prev_id` it does not recognise, it must resynchronise its list by calling the `/user/keys/query` API and resume the process

It seems to me that the endpoint that should be used to resync device lists is `/_matrix/federation/v1/user/devices/{userId}`; it's also the one Synapse has always been using afaict.

Start at the device management section of the Matrix server-server specification and compare its resynchronisation instruction with the surrounding federation API definitions. Verify which endpoint is intended, then update the specification reference so it matches the documented device-list resync API.

Written by the indexing model from the issue text.