text: Title: What is the purpose of parse_obj's second argument: "update"?
  Body: ### First Check
  
  - [X] I added a very descriptive title to this issue.
  - [X] I used the GitHub search to find a similar issue and didn't find it.
  - [X] I searched the SQLModel documentation, with the integrated search.
  - [X] I already searched in Google "How to X in SQLModel" and didn't find any information.
  - [X] I already read and followed all the tutorial in the docs and didn't find an answer.
  - [X] I already checked if it is not related to SQLModel but to [Pydantic](https://github.com/samuelcolvin/pydantic).
  - [X] I already checked if it is not related to SQLModel but to [SQLAlchemy](https://github.com/sqlalchemy/sqlalchemy).
  
  ### Commit to Help
  
  - [X] I commit to help with one of those options 👆
  
  ### Example Code
  
  ```python
  SomeModel.parse_obj({"one": 2}, {"three": 4})
  ```
  
  
  ### Description
  
  SQLModel's `parse_obj` supports the update argument which is not supported by pydantic's BaseModel. However, I do not see it ever used in docs, github issues, or in source code.
  
  Could someone explain why it was added? What was the actual use case? I understand what it does, but I don't understand why it does it.
  
  ### Operating System
  
  Linux, Windows, macOS, Other
  
  ### Operating System Details
  
  _No response_
  
  ### SQLModel Version
  
  0.0.8
  
  ### Python Version
  
  3.11
  
  ### Additional Context
  
  _No response_