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

[Luke Lau via llvm-commits]

lukel97 wrote:

> I have written an internel doc about this before, and this is how I organized the content: Hopefully this can help us to perfect this doc. :-)

Nice! Good idea covering early-clobber and TIED pseudos, they probably warrant some documentation too. 

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