[llvm] [docs] [RISCV] Update docs regarding RV32E/RV64E (PR #170707)

[Sam Elliott via llvm-commits]

lenary wrote:

I'm not sure these are "fully supported". iirc the ABIs are unstable, and i think there are some compiler features that don't work with rv32e/rv64e because they require using a higher register (t3?)

Maybe we need a separate section listing out limitations for rv32e/rv64e?

https://github.com/llvm/llvm-project/pull/170707