[llvm] [Docs][RISCV] Document RISC-V vector codegen (PR #96740)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

Thanks! It is inconsistent whether whiches deserve a comma. A glossary at the bottom would be nice. What is AVL?

https://github.com/llvm/llvm-project/pull/96740