Proposed re-wording on help text for --jobs

[eminence]

This is a continuation of rust-lang/rust#41997, see that issue for a small bit of background.

I was confused by the cargo test --help text for the --jobs option. Here is a proposal for a slightly re-worded text that might make is more immediately clear what --jobs is used for:

Usage: 
    cargo test [options] [--] [<args>...]

Options:
    -j N, --jobs N               Number of parallel builds, see below for details


The --jobs argument affects the building of the test executable but does
not affect how many jobs are used when running the tests. The default value for
the --jobs argument is the number of CPUs. If you want to control the number of
simultaneous running test cases, pass the test-threads option to the test binaries:

    cargo test -- --test-threads

[alexcrichton]

Sounds good to me!