[site] more edits

site/src/docs/running.md

@@ -196,12 +196,22 @@ cargo nextest run -E 'platform(host)'
 `-jN`, `--test-threads=N`
 : Number of tests to run simultaneously. Note that this is separate from the number of build jobs to run simultaneously, which is specified by `--build-jobs`.
 
+  `N` can be:
+
+  * `num-cpus` to run as many tests as the amount of [available parallelism] (typically the number of CPU hyperthreads). This is the default.
+  * a positive integer (e.g. `8`) to run that many tests simultaneously.
+  * a negative integer (e.g. `-2`) to run available parallelism minus that many tests simultaneously. For example, on a machine with 8 CPU hyperthreads, `-2` would run 6 tests simultaneously.
+
+  Tests can be marked as taking up more than one available slot. For more, see [*Heavy tests and `threads-required`*](configuration/threads-required.md).
+
 `--run-ignored=only` <!-- md:version 0.9.76 -->
-: Run only ignored tests. (With prior nextest versions, use `--run-ignored ignored-only`.)
+: Run only ignored tests. (With prior nextest versions, use `--run-ignored=ignored-only`.)
 
 `--run-ignored=all`
 : Run both ignored and non-ignored tests.
 
+[available parallelism]: https://doc.rust-lang.org/std/thread/fn.available_parallelism.html
+
 ## Controlling nextest's output
 
 For information about configuring the way nextest displays its human-readable output, see [_Reporting test results_](reporting.md).