[llvm] [RISCV] Add documentation that Zvbc and Zvk* are supported through intrinsics. NFC (PR #108577)

[Brandon Wu via llvm-commits]

4vtomat wrote:

> Do we also need to change `zvbb` to supported in this patch?
> Or it can be in another patch?

Oh I just saw it in another patch!

https://github.com/llvm/llvm-project/pull/108577