Expand documentation on timeouts (#1204)

Make it clear that a timeout does not prevent long running tests.
diff --git a/pkgs/test/README.md b/pkgs/test/README.md
index 63ea8be..21b5ea3 100644
--- a/pkgs/test/README.md
+++ b/pkgs/test/README.md
@@ -615,9 +615,12 @@
 
 ### Timeouts
 
-By default, tests will time out after 30 seconds of inactivity. However, this
-can be configured on a per-test, -group, or -suite basis. To change the timeout
-for a test suite, put a `@Timeout` annotation at the top of the file:
+By default, tests will time out after 30 seconds of inactivity. The timeout
+applies to deadlocks or cases where the test stops making progress, it does not
+ensure that an overall test case or test suite completes within any set time.
+
+Timeouts can be configured on a per-test, -group, or -suite basis. To change the
+timeout for a test suite, put a `@Timeout` annotation at the top of the file:
 
 ```dart
 @Timeout(const Duration(seconds: 45))