parallelism: job slots vs. levels

[Dan Jacobson]

Anyways,
$ make x& make y& wait
cannot always be rewritten with -j.
$ make -j[whatever number] x y
will act differently except for special cases of x and y;
probably when both x and y have no dependencies.

Anyways, with -j examples added to the manual, we would get on the
right track about how to use -j.