win32: add comment about WinError

Prevent reintroducing the bug that was added in e34106fa0dc3 (and fixed with
a2285e2fc949).

mercurial/win32.py

@@ -279,7 +279,7 b' def executablepath():'
     buf = ctypes.create_string_buffer(size + 1)
     len = _kernel32.GetModuleFileNameA(None, ctypes.byref(buf), size)
     if len == 0:
-        raise ctypes.WinError()
+        raise ctypes.WinError() # Note: WinError is a function
     elif len == size:
         raise ctypes.WinError(_ERROR_INSUFFICIENT_BUFFER)
     return buf.value