Docs: Explain t3.chats usage of context

Explain in detail:

• what does t3.chat add to the context

  • as default, unchangeable by the user

  • using the settings in user profile

• when forking a thread
  i.e. how much of the source chat is copied
  is anything added beyond that

• when changing the model
  see above

I got curios because one model keeps referring to information i once gave in my profile but deleted later. I got the impression, that information “leaked” between chats.