diff --git a/docs/source/conf.py b/docs/source/conf.py
index eb278bf65f..a1df00736b 100755
--- a/docs/source/conf.py
+++ b/docs/source/conf.py
@@ -172,8 +172,9 @@ def autodoc_process_signature(
 
 # currently undocumented things
 logger = getLogger("trio")
-UNDOCUMENTED = {
-    "trio._subprocess.HasFileno.fileno",
+# ``ParkingLot.broken_by`` is a slotted attrs field; its documentation lives
+# in the ``ParkingLot`` class docstring's ``Attributes:`` block.
+UNDOCUMENTED: set[str] = {
     "trio.lowlevel.ParkingLot.broken_by",
 }
 
diff --git a/docs/source/reference-lowlevel.rst b/docs/source/reference-lowlevel.rst
index 077f21cd27..d2da0ae05d 100644
--- a/docs/source/reference-lowlevel.rst
+++ b/docs/source/reference-lowlevel.rst
@@ -482,6 +482,7 @@ Wait queue abstraction
 .. autoclass:: ParkingLot
    :members:
    :undoc-members:
+   :exclude-members: broken_by
 
 .. autoclass:: ParkingLotStatistics
    :members:
diff --git a/newsfragments/3221.doc.rst b/newsfragments/3221.doc.rst
new file mode 100644
index 0000000000..319649286e
--- /dev/null
+++ b/newsfragments/3221.doc.rst
@@ -0,0 +1,3 @@
+Filled in the missing documentation for ``trio._subprocess.HasFileno.fileno``
+and ``trio.lowlevel.ParkingLot.broken_by``, which completes the checklist in
+issue #3221.
diff --git a/src/trio/_core/_parking_lot.py b/src/trio/_core/_parking_lot.py
index ddf6276117..3f4d6bb710 100644
--- a/src/trio/_core/_parking_lot.py
+++ b/src/trio/_core/_parking_lot.py
@@ -146,6 +146,10 @@ class ParkingLot:
     the number of parked tasks, and ``if parking_lot: ...`` to check whether
     there are any parked tasks.
 
+    Attributes:
+        broken_by: The tasks that broke this parking lot, most recent
+            last. See :meth:`break_lot`.
+
     """
 
     # {task: None}, we just want a deque where we can quickly delete random
diff --git a/src/trio/_subprocess.py b/src/trio/_subprocess.py
index d73ba3dc23..9d5dbc7cc8 100644
--- a/src/trio/_subprocess.py
+++ b/src/trio/_subprocess.py
@@ -98,7 +98,12 @@ def pidfd_open(fd: int, flags: int) -> int:
 class HasFileno(Protocol):
     """Represents any file-like object that has a file descriptor."""
 
-    def fileno(self) -> int: ...
+    def fileno(self) -> int:
+        """Returns the file descriptor of this file-like object, as an
+        integer. It is passed to the child process, e.g. when a
+        :class:`HasFileno` object is used as ``stdin``, ``stdout`` or
+        ``stderr`` of a spawned process."""
+        ...
 
 
 @final
diff --git a/src/trio/_tests/_check_type_completeness.json b/src/trio/_tests/_check_type_completeness.json
index d8a2948cd9..a44f90dfe0 100644
--- a/src/trio/_tests/_check_type_completeness.json
+++ b/src/trio/_tests/_check_type_completeness.json
@@ -12,7 +12,6 @@
     "all": [
         "No docstring found for class \"trio._core._run.Task\"",
         "No docstring found for class \"trio._socket.SocketType\"",
-        "No docstring found for function \"trio._subprocess.HasFileno.fileno\"",
         "No docstring found for class \"trio._sync.AsyncContextManagerMixin\"",
         "No docstring found for function \"trio._sync._HasAcquireRelease.acquire\"",
         "No docstring found for function \"trio._sync._HasAcquireRelease.release\"",