Documentation: riscv: Remove KPROBES_ON_FTRACE

Since commit 7caa9765 ("ftrace: riscv: move from REGS to ARGS"),
kprobe on ftrace is not supported by riscv.

And commit 33081722 ("trace: riscv: Remove deprecated kprobe on
ftrace support") removed the relevant code, but left out the
documentation, so fix that.

Signed-off-by: Chen Pei <cp0613@linux.alibaba.com>
Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250108020342.4172-1-cp0613@linux.alibaba.com