Document issue upgrading between Connect packages

As neither rpi-connect 1.4.0 nor rpi-connect-lite 1.4.0 clean up their
systemd symlinks or stop their services on uninstall, it is important
that users upgrade from rpi-connect 1.4.0 to rpi-connect 2.0.0 or they
upgrade from rpi-connect-lite 1.4.0 to rpi-connect-lite 2.0.0.

If they attempt to switch packages during the upgrade (i.e. go from
rpi-connect 1.4.0 to rpi-connect-lite 2.0.0 or from rpi-connect-lite
1.4.0 to rpi-connect 2.0.0), the necessary migration script will not run
leaving them in an inconsistent state until they either purge their old
package or upgrade it.

Document this issue by encouraging users to upgrade their existing
package in place before any attempt to switch between rpi-connect and
rpi-connect-lite.

Author: mudge

documentation/asciidoc/services/connect/troubleshooting.adoc

@@ -10,6 +10,27 @@
 
 * Browsers intercept certain keys and key combinations. As a result, you can't type these keys into your Connect window. Screen sharing includes a toolbar to simulate some of the most popular intercepted keys.
 
+* When upgrading from version 1 to version 2, ensure to upgrade whichever package you currently have installed before switching between rpi-connect and rpi-connect-lite. This will ensure Connect's services are migrated to the new format, i.e. if you currently have rpi-connect installed:
++
+[source,console]
+----
+$ sudo apt install --only-upgrade rpi-connect
+----
++
+If you have rpi-connect-lite installed:
++
+[source,console]
+----
+$ sudo apt install --only-upgrade rpi-connect-lite
+----
++
+If successful, you should see the following output during the upgrade:
++
+[source,console]
+----
+Replacing globally-enabled rpi-connect services with user-enabled ones...
+----
+
 === Common issues
 
 ==== Screen sharing not available