docs: use virtual env for upgrade; stop cron jobs.

Author: rouilj

doc/admin_guide.txt

@@ -1352,10 +1352,14 @@ Steps you may take:
 
 2. If you're using an RDBMS backend, make a backup of its contents now.
 3. Make a backup of the tracker home itself.
-4. Stop the tracker web and email frontends.
-5. Install the new version of the software::
-
-    python setup.py install
+4. Stop the tracker web, email frontends and any scheduled
+   (cron) jobs that access the tracker.
+5. Install the new version of the software in a new virtual
+   environment::
+
+    python3 -m venv /path/to/env
+    . /path/to/env/bin/activate
+    python3 pip install roundup
 
 6. Follow the steps in the `upgrading documentation`_ for all the
    versions between your original version and the new version.