fix(doc): improve test runner timeout docs

Add a warning to clarify that it is not a reliable mechanism for
cancelling tests.

Fix the default value of the timeout option.

Simplify and capitalize the YAML description field.

PR-URL: nodejs/node#43836
Reviewed-By: Antoine du Hamel <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
(cherry picked from commit 545ecc57360fad50c49c81b56ae25fcc99d38456)

Author: tniessen

Diff for: README.md

@@ -341,7 +341,7 @@ internally.
     the test is `TODO`. **Default:** `false`.
   - `timeout` {number} A number of milliseconds the test will fail after.
     If unspecified, subtests inherit this value from their parent.
-    **Default:** `30_000`.
+    **Default:** `Infinity`.
 - `fn` {Function|AsyncFunction} The function under test. The first argument
   to this function is a [`TestContext`][] object. If the test uses callbacks,
   the callback function is passed as the second argument. **Default:** A no-op
@@ -374,6 +374,11 @@ test('top level test', async t => {
 })
 ```
 
+The `timeout` option can be used to fail the test if it takes longer than
+`timeout` milliseconds to complete. However, it is not a reliable mechanism for
+canceling tests because a running test might block the application thread and
+thus prevent the scheduled cancellation.
+
 ## `describe([name][, options][, fn])`
 
 * `name` {string} The name of the suite, which is displayed when reporting test
@@ -488,7 +493,7 @@ execution of the test function. This function does not return a value.
     the test is `TODO`. **Default:** `false`.
   - `timeout` {number} A number of milliseconds the test will fail after.
     If unspecified, subtests inherit this value from their parent.
-    **Default:** `30_000`.
+    **Default:** `Infinity`.
 - `fn` {Function|AsyncFunction} The function under test. The first argument
   to this function is a [`TestContext`][] object. If the test uses callbacks,
   the callback function is passed as the second argument. **Default:** A no-op