message: 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
diff: @@ -56,7 +56,7 @@ Options:
  --no-run Compile, but don't run tests
  -p SPEC, --package SPEC ... Package to run tests for
  --all Test all packages in the workspace
  - -j N, --jobs N Number of parallel jobs, defaults to # of CPUs
  + -j N, --jobs N Number of parallel builds, see below for details
  --release Build artifacts in release mode, with optimizations
  --features FEATURES Space-separated list of features to also build
  --all-features Build all available features
  @@ -86,7 +86,12 @@ All packages in the workspace are tested if the `--all` flag is supplied. The
  `--all` flag may be supplied in the presence of a virtual manifest.
  The --jobs argument affects the building of the test executable but does
  -not affect how many jobs are used when running the tests.
  +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=1
  Compilation can be configured via the `test` profile in the manifest.
  