[llvm][Release] Add note about binaries to Github release description (#69698)

This appears on the announcements e.g.
https://discourse.llvm.org/t/llvm-17-0-3-released/74172 and it is
important context.

However a lot of folks see release pages e.g.
https://github.com/llvm/llvm-project/releases/tag/llvmorg-17.0.3 first
so it's good to include it there too.