[Documentation] Document the error handling of the standard library

The following example executes fine without raising an error:

from concurrent.futures import ProcessPoolExecutor


def raise_error():
    raise ValueError()

if __name__ == "__main__":
    with ProcessPoolExecutor() as exe:
        fs = exe.submit(raise_error)
        print(fs.done())
        print(fs.exception() is not None)
        # print(fs.result())
    print("done")

Only, when the fs.result() call is not commented out, then the ValueError() is raised. The print out here is False, True, done.

Start with the ProcessPoolExecutor example in the issue and inspect the repository's existing documentation structure to find the appropriate location. Document when the submitted exception is exposed through Future.exception() versus raised by Future.result(), including the observed output and behavior shown in the example.