[clang] [analyzer][docs] Further improve the optin.core.FixedAddressDereference docs (PR #182037)

[Balázs Benics via cfe-commits]

steakhal wrote:

I noticed that the attribute was just merged, so I decided to xref to it. It properly links to the attr doc paragraph.
<img width="1127" height="269" alt="image" src="https://github.com/user-attachments/assets/53eb0f87-bafa-4e66-908c-9ceb9cf1488f" />


https://github.com/llvm/llvm-project/pull/182037