Explain coverage pragmas

src/trio/_core/_tests/test_thread_cache.py

@@ -221,14 +221,21 @@ def foo() -> None:
         # if this test fails, this will hang, triggering a timeout.
         os.waitpid(child_pid, 0)
     else:
+        # this is necessary because os._exit doesn't unwind the stack,
+        # so coverage doesn't get to automatically stop and save
+        # coverage information.
         try:
             import coverage
 
             cov = coverage.Coverage.current()
+            # the following pragmas are necessary due to not having
+            # necessary test runs (w/ coverage but not running under
+            # it, w/o coverage)
+
             if cov:  # pragma: no branch
                 cov.stop()
                 cov.save()
         except ImportError:  # pragma: no cover
             pass
 
-        os._exit(0)  # pragma: no cover
+        os._exit(0)  # pragma: no cover  # coverage was stopped above.