Reword the `cargo test --help` text about the --jobs argument
Reword the `cargo test --help` text about the --jobs argument

This should reduce confusion about what the --jobs argument does and how
to control the number of simultaneous running test cases.

Closes #4045

eminence/cargo,rust-lang/cargo,nipunn1313/cargo,tsurai/cargo,vojtechkral/cargo,eminence/cargo,eminence/cargo,achanda/cargo,tsurai/cargo,achanda/cargo,rust-lang/cargo,nipunn1313/cargo,tshepang/cargo,rustyhorde/cargo,tsurai/cargo,rustyhorde/cargo,rust-lang/cargo,durka/cargo,achanda/cargo,achanda/cargo,tshepang/cargo,rust-lang/cargo,tshepang/cargo,durka/cargo,tshepang/cargo,vojtechkral/cargo,tsurai/cargo,durka/cargo,nipunn1313/cargo,vojtechkral/cargo,eminence/cargo,vojtechkral/cargo,durka/cargo,nipunn1313/cargo,rustyhorde/cargo,tsurai/cargo,rustyhorde/cargo