docs: update test concurrency description / default values

PR-URL: nodejs/node#46457
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jacob Smith <jacob@frende.me>
(cherry picked from commit 7d68b7bbfc9ffbd2ad0913972ac0b1a315679b06)

README.md

@@ -597,11 +597,10 @@ added: REPLACEME
   properties are supported:
   * `concurrency` {number|boolean} If a number is provided,
     then that many files would run in parallel.
-    If truthy, it would run (number of cpu cores - 1)
-    files in parallel.
-    If falsy, it would only run one file at a time.
-    If unspecified, subtests inherit this value from their parent.
-    **Default:** `true`.
+    If `true`, it would run `os.availableParallelism() - 1` test files in
+    parallel.
+    If `false`, it would only run one test file at a time.
+    **Default:** `false`.
   * `files`: {Array} An array containing the list of files to run.
     **Default** matching files from [test runner execution model][].
   * `signal` {AbortSignal} Allows aborting an in-progress test execution.
@@ -631,10 +630,9 @@ run({ files: [path.resolve('./tests/test.js')] })
   properties are supported:
   - `concurrency` {number|boolean} If a number is provided,
     then that many tests would run in parallel.
-    If truthy, it would run (number of cpu cores - 1)
-    tests in parallel.
+    If `true`, it would run `os.availableParallelism() - 1` tests in parallel.
     For subtests, it will be `Infinity` tests in parallel.
-    If falsy, it would only run one test at a time.
+    If `false`, it would only run one test at a time.
     If unspecified, subtests inherit this value from their parent.
     **Default:** `false`.
   - `only` {boolean} If truthy, and the test context is configured to run
@@ -1335,9 +1333,12 @@ execution of the test function. This function does not return a value.
   `fn` does not have a name.
 - `options` {Object} Configuration options for the subtest. The following
   properties are supported:
-  - `concurrency` {number} The number of tests that can be run at the same time.
+  - `concurrency` {number|boolean|null} If a number is provided,
+    then that many tests would run in parallel.
+    If `true`, it would run all subtests in parallel.
+    If `false`, it would only run one test at a time.
     If unspecified, subtests inherit this value from their parent.
-    **Default:** `1`.
+    **Default:** `null`.
   - `only` {boolean} If truthy, and the test context is configured to run
     `only` tests, then this test will be run. Otherwise, the test is skipped.
     **Default:** `false`.