Changes in SignonAuthSession: libsignon-glib Reference Manual

## Changes in SignonAuthSession

The long deprecated `signon_auth_session_process()` has been removed and the `signon_auth_session_process_async()` method has been renamed to `signon_auth_session_process()` as asynchronous methods are the default way to use this API.

`signon_auth_session_query_available_mechanisms()` has been removed and can be replaced by `signon_auth_service_get_mechanisms()` with a manual filtering if needed.