unittest: add clarifying comment on unittest.SkipTest -> pytest.skip code

I was tempted to remove it, until I figured out why it was there.

src/_pytest/unittest.py

@@ -343,6 +343,10 @@ def pytest_runtest_makereport(item: Item, call: CallInfo[None]) -> None:
             except AttributeError:
                 pass
 
+    # Convert unittest.SkipTest to pytest.skip.
+    # This is actually only needed for nose, which reuses unittest.SkipTest for
+    # its own nose.SkipTest. For unittest TestCases, SkipTest is already
+    # handled internally, and doesn't reach here.
     unittest = sys.modules.get("unittest")
     if (
         unittest
@@ -350,7 +354,6 @@ def pytest_runtest_makereport(item: Item, call: CallInfo[None]) -> None:
         and isinstance(call.excinfo.value, unittest.SkipTest)  # type: ignore[attr-defined]
     ):
         excinfo = call.excinfo
-        # Let's substitute the excinfo with a pytest.skip one.
         call2 = CallInfo[None].from_call(
             lambda: pytest.skip(str(excinfo.value)), call.when
         )