Add documentation note about upgrading multiple pipx installations of borgmatic.

borgmatic/execute.py

@@ -134,6 +134,7 @@ def log_outputs(processes, exclude_stdouts, output_log_level, borg_local_path):
                 still_running = True
 
             command = process.args.split(' ') if isinstance(process.args, str) else process.args
+
             # If any process errors, then raise accordingly.
             if exit_code_indicates_error(command, exit_code, borg_local_path):
                 # If an error occurs, include its output in the raised exception so that we don't

docs/how-to/upgrade.md

@@ -16,7 +16,9 @@ is to run the following:
 sudo pipx upgrade borgmatic
 ```
 
-(Or without `sudo` if you installed borgmatic as a non-root user.)
+Omit `sudo` if you installed borgmatic as a non-root user. And if you
+installed borgmatic *both* as root and as a non-root user, you'll need to
+upgrade each installation indepedently.
 
 If you originally installed borgmatic with `sudo pip3 install --user`, you can
 uninstall it first with `sudo pip3 uninstall borgmatic` and then [install it