Docs for vendored AI-powered features

What does this MR do and why?

Now when the following MR/issue merged/closed:

• Support mixed vendored and self-hosted models (!164924 - merged)
• Support a mix of self-hosted and GitLab-hosted ... (#463917 - closed)

We can document the behavior behind this option:

It means:

• When AI Vendor option is selected, the requests will be sent to Cloud Connector
• CLOUD_CONNECTOR_SELF_SIGN_TOKENS is redundant and the recommendation to set it can be removed