clarify that installing -py3 removes the old pkg

debian/NEWS

 matrix-synapse-py3 (0.34.0) stable; urgency=medium
 
   matrix-synapse-py3 is intended as a drop-in replacement for the existing
-  matrix-synapse package. The replacement should be relatively seamless,
+  matrix-synapse package. When the package is installed, matrix-synapse will
+  be automatically uninstalled. The replacement should be relatively seamless,
   however, please note the following important differences to matrix-synapse:
 
   * Most importantly, the matrix-synapse service now runs under Python 3 rather
@@ -9,12 +10,12 @@ matrix-synapse-py3 (0.34.0) stable; urgency=medium
 
   * Synapse is installed into its own virtualenv (in /opt/venvs/matrix-synapse)
     instead of using the system python libraries. (This may mean that you can
-    remove a number of old dependencies with `apt-get autoremove`).
+    remove a number of old dependencies with `apt autoremove`).
 
   matrix-synapse-py3 will take over responsibility for the existing
   configuration files, including the matrix-synapse systemd service.
 
-  Beware, however, that `apt-get purge matrix-synapse` will *disable* the
+  Beware, however, that `apt purge matrix-synapse` will *disable* the
   matrix-synapse service (so that it will not be started on reboot), even
   though that service is no longer being provided by the matrix-synapse
   package. It can be re-enabled with `systemctl enable matrix-synapse`.