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

[Luke Lau via llvm-commits]

lukel97 wrote:

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

The AVL is the application vector length, i.e. the operand to vsetvli. I think it's quite easily confused with VL, which is the actual register. Those _VL nodes should probably be called _AVL nodes, and likewise everywhere where we have a "VL" value. The VL register isn't guaranteed to end up as that value, it's just what RISCVInsertVSETVLI will use the vsetvli's AVL.

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