[clang] [C2y] Add documentation to conform to WG14 N3262; NFC (PR #98146)

[John McCall via cfe-commits]

rjmccall wrote:

Ah right, I'd forgotten that some ABIs use that array trick to get it to pass by reference, and you're right that that makes it ill-formed to simply assign around.

I like your idea of specifically making it UB to copy with `memcpy` etc and just advising that people use va_copy.

https://github.com/llvm/llvm-project/pull/98146