doc: clarify concurrency model of test runner

Refs: nodejs#47365

doc/api/test.md

@@ -379,6 +379,11 @@ Otherwise, the test is considered to be a failure. Test files must be
 executable by Node.js, but are not required to use the `node:test` module
 internally.
 
+Each test file is executed as if it was a regular script. That is, if the test
+file itself uses `node:test` to define tests, all of those tests will be
+executed within a single application thread, regardless of the value of the
+`concurrency` option of [`test()`][].
+
 ## Collecting code coverage
 
 > Stability: 1 - Experimental
@@ -731,7 +736,8 @@ added:
 * `options` {Object} Configuration options for running tests. The following
   properties are supported:
   * `concurrency` {number|boolean} If a number is provided,
-    then that many files would run in parallel.
+    then that many test processes would run in parallel, where each process
+    corresponds to one test file.
     If `true`, it would run `os.availableParallelism() - 1` test files in
     parallel.
     If `false`, it would only run one test file at a time.
@@ -795,7 +801,7 @@ changes:
 * `options` {Object} Configuration options for the test. The following
   properties are supported:
   * `concurrency` {number|boolean} If a number is provided,
-    then that many tests would run in parallel.
+    then that many tests would run in parallel within the application thread.
     If `true`, it would run `os.availableParallelism() - 1` tests in parallel.
     For subtests, it will be `Infinity` tests in parallel.
     If `false`, it would only run one test at a time.
@@ -1765,8 +1771,8 @@ changes:
   `fn` does not have a name.
 * `options` {Object} Configuration options for the subtest. The following
   properties are supported:
-  * `concurrency` {number|boolean|null} If a number is provided,
-    then that many tests would run in parallel.
+  * `concurrency` {number|boolean} If a number is provided,
+    then that many tests would run in parallel within the application thread.
     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.