target/riscv/cpu.c: do better with 'named features' doc

target/riscv/cpu.c: do better with 'named features' doc

Most of the named features are added directly in isa_edata_arr[], some
of them are also added in riscv_cpu_named_features(). There is a reason
for that, and the existing docs can do better explaining it.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20250529202315.1684198-4-dbarboza@ventanamicro.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250604174329.1147549-4-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>