Adding a warning/info message when nothing was parallelized successfully

It might be a good idea to add a few warning/info messages to help set user expectations when running PaSh. In particular to give some intuition to the user about why they didn't get any benefits through PaSh. Some categories that I can think of which are highly relevant are:
1. No part of the input script was parallelized. Do you have annotations for the commands in your scripts? Are you using many custom functions?
2. The script execution was very short (<5s) to see benefits with PaSh. Maybe try running a script that takes longer?
3. Some fragments of the input script were parallelized, but none of them took more than 1s of execution so PaSh hasn't managed to speed things up.
We could follow up all these messages with a suggestion to use -d 1 to see more about what was parallelized and what not and get some guidance into the issues.

Start by tracing PaSh's execution and reporting path, then inspect the existing -d 1 output to see what parallelization and fragment timing information is available. Define how the three no-benefit cases are detected and where their user-facing messages should appear; done means each applicable case gives a clear explanation and points users to -d 1.

Written by the indexing model from the issue text.