docs: update concurrency wording per reviewer suggestion

Author: azadgupta1

doc/api/test.md

@@ -1619,7 +1619,7 @@ 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 concurrently within the test process.
+    then that many tests would run asynchronously (they are still managed by the single-threaded event loop).
     If `true`, all scheduled asynchronous tests run concurrently within the
     thread. If `false`, only one test runs at a time.
     If unspecified, subtests inherit this value from their parent.
@@ -3882,7 +3882,7 @@ changes:
 * `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 concurrently within the test process.
+    then that many tests would run asynchronously (they are still managed by the single-threaded event loop).
     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.