Add a hint that coverage combine might be needed

[nedbat]

From the Python IRC channel on Libera:

Person: coverage.py's `coverage report` gives me "No data to report.", and it looks like that is because the output file isn't .coverage, but dot-suffixed with my machine name etc. Now, this appears to be done *not* because of `parallel = true` in my config, but because `concurrency = multiprocessing`. However, `coverage report` does not pick up on this, or even when I do explicitly add `parallel = true`
Person: If I specify the output file explicitly when calling `coverage report`, I get the desired output (but that's not a feasible solution as these filenames are not consistent)
Person: fml.... I type out my problem and the solution appears: yes, I have to call `coverage combine` explicitly

The "No data to report" message could check if there are dot-suffixed data files, and add, "Perhaps you need to run coverage combine" or something similar. This would guide people to a solution.