Make document setter private

Author: NoahStapp

bson/errors.py

@@ -44,8 +44,7 @@ def document(self) -> Any:
         ..versionadded:: 4.16"""
         return self._document
 
-    @document.setter
-    def document(self, value: Any) -> None:
+    def _set_document(self, value: Any) -> None:
         self._document = value