Fix custom prompts configuration manual.md

[cyun9601]

If you make custom prompt work as per #486, you should write the output parser.

Additionally, it seems that need to list all the items available as config for prompts in this user guide file.

[AsmaaMHadir]

I am getting the same error running the ConversationalRetrievalChain.from_llm object. I'm researching a potential workaround/solution