docs: clarify subtests progress output

changelog/13902.doc.rst

@@ -0,0 +1 @@
+Clarified how subtest progress markers are shown in the documentation.

doc/en/how-to/subtests.rst

@@ -26,7 +26,7 @@ Subtests are an alternative to parametrization, particularly useful when the exa
 
 Each assertion failure or error is caught by the context manager and reported individually:
 
-.. code-block:: pytest
+.. code-block:: text
 
     $ pytest -q test_subtest.py
     uuuuuF                                                               [100%]
@@ -63,6 +63,8 @@ Each assertion failure or error is caught by the context manager and reported in
 
 In the output above:
 
+* The compact progress output uses ``u`` for both passed and failed subtests;
+  see the short test summary for each failed subtest.
 * Subtest failures are reported as ``SUBFAILED``.
 * Subtests are reported first and the "top-level" test is reported at the end on its own.