Document how XR connection details work in v2

[jbw976]

What's Missing?

There's a bit of confusion, and out of date documentation, for how connection details work for XRs in v2.

Some of the problems are captured in crossplane/crossplane#6837 (comment).

TL;DR, we removed connection details from XRs in v2, but we don't have good documentation and guidance on how to handle that. Nic's suggestion in crossplane/crossplane#6440 is the only "doc" i know of:

I feel comfortable dropping this functionality (from core) for v2 XRs because in v2 you can recreate it using functions - just have your XR compose a secret with the XR's connection details in it.

We should perhaps consider surfacing this more clearly as a breaking change too, depending on the scope of impact once that is better assessed.