[Obs AI Assistant] Add documentation for "user instructions" feature

[sorenlouv]

User instructions were added in 8.16 (elastic/kibana#187607). They allow end users to append instruction to the system prompt. This way they can get the LLM to consistently behave in a specific way.

One user instruction can be created per user. It can be modified by going to Knowledge Base -> "Edit user-specific prompt".

[sorenlouv]

@mdbirnstiehl Is this something you can help with?

[mdbirnstiehl]

@mdbirnstiehl Is this something you can help with?

Hi @sorenlouv, sorry for the delayed response. We are in the middle of a docs migration which is taking up a lot of our time, so we can help, but it might take a little time. I'll mention @bmorelli25 here just so it's on our radar and we can prioritize it if need be.