Document RISC-V d, e, and f extensions

These are being stabilized in
https://github.com/rust-lang/rust/pull/156188 as cfg-only features. It
is a little odd to list them here in the target_feature attribute
section since they can't be used with the attribute, but I think it
makes sense to keep things together. Perhaps in the future we may
consider breaking these lists out into a separate section and rearrange
things.