Documentation or Examples about proxyUrl, provideClientKey and signClientKey endpoints?

There is minimal documentation about these 3 endpoints in the AP specs:

proxyUrl
Endpoint URI so this actor’s clients may access remote ActivityStreams objects which require authentication to access. To use this endpoint, the client posts an x-www-form-urlencoded id parameter with the value being the id of the requested ActivityStreams object.

provideClientKey
If Linked Data Signatures and HTTP Signatures are being used for authentication and authorization, this endpoint specifies a URI at which browser-authenticated users may authorize a client’s public key for client to server interactions.

signClientKey
If Linked Data Signatures and HTTP Signatures are being used for authentication and authorization, this endpoint specifies a URI at which a client key may be signed by the actor’s key for a time window to act on behalf of the actor in interacting with foreign servers.

They however seem fundamental pieces for a proper Client-to-Server AP implementation. Does anyone have more informations about this, like examples of what these endpoints are supposed to accept and what they should return ? I could not find any specs about that.

For the proxyUrl endpoint, if the URI to be requested is provided via x-www-form-urlencoded, does it mean it only works for GET operations ? We would need it for POST, PATCH, PUT, DELETE operations as well, as we have implemented the Linked Data Plateform (LDP) protocol which allow all of them.

Thanks !