doc: clarify basic upgrade instructions to target single instance

Author: rouilj

doc/upgrading.txt

@@ -33,12 +33,15 @@ General steps:
       versions starting at 1.5 and ending at 2.3. E.G.
       1.5.0 -> 1.5.1, 1.5.1 -> 1.6.0, ..., 2.1.0 -> 2.2.0,
       2.2.0 -> 2.3.0.
-   6. Run ``roundup-admin -i <tracker_home> migrate`` using the newer
-      version of Roundup for **all** the trackers you have
-      upgraded. This will update the database if it is required.
+   6. Run ``roundup-admin -i <tracker_home> migrate`` using
+      the newer version of Roundup for the instance you are
+      upgrading. This will update the database if it is
+      required.
    7. Bring your Roundup instance back online
    8. Test
 
+Repeat for each tracker instance.
+
 .. note::
    The v1.5.x releases of Roundup were the last to support
    Python v2.5 and v2.6.  Starting with the v1.6 releases of Roundup