add comment about PYOPCUA_NO_TYPO_CHECK

opcua/uatypes.py

@@ -250,6 +250,10 @@ class _FrozenClass(object):
         object.__setattr__(self, key, value)
 
 if "PYOPCUA_NO_TYPO_CHECK" in os.environ:
+    # typo check is cpu consuming, but it will make debug easy.
+    # if typo check is not need (in production), please set env PYOPCUA_NO_TYPO_CHECK.
+    # this will make all uatype class inherit from object intead of _FrozenClass
+    # and skip the typo check.
     FrozenClass = object
 else:
     FrozenClass = _FrozenClass