[clang] [clang][RISCV] Update vcpop.v C interface to follow the nameing convention (PR #94318)

[Brandon Wu via cfe-commits]

4vtomat wrote:

> Could you give few more word on the description to mention we missed that in the vector crpyto intrinsic proposal, and it's fixing but rather than incompatible/breaking change for the intrinsic API?

Updated description. We are missing `vcpop.v` in the rvv_intrinsic_doc, so I think we don't break anything lol~

https://github.com/llvm/llvm-project/pull/94318