Clarify --warn-unreachable behavior for pass statements

Author: brovoski69

docs/source/command_line.rst

@@ -535,6 +535,15 @@ potentially problematic or redundant in some way.
             else:
                 # Error: 'Statement is unreachable' error
                 print(x + "bad")
+    Note that unreachable ``pass`` statements are intentionally not
+    reported by this option. This allows ``pass`` to be used as a
+    placeholder during development without triggering an error.For
+    example::
+
+       def f(x: int) -> None:
+           if x > 0:
+               return
+               pass  # no --warn-unreachable error is reported here
 
     To help prevent mypy from generating spurious warnings, the "Statement is
     unreachable" warning will be silenced in exactly two cases: