[Docs] Add version support information for opaque pointers (NFC)

I've seen a few people try to enable opaque pointers with LLVM 14
already. While LLVM 14 has pretty good baseline support, there are
enough missing pieces that you're definitely going to hit assertion
failures if you try this.

Add some wording to make it clear what the support (or planned
support) for opaque/typed pointers is across LLVM 14, 15, and 16.