docs: Downgrade arm64 & riscv from titles to comment

Remove the title string ("====") from under arm64 & riscv and move them
to a commment under the perf_user_access sysctl. They are explanations,
*not* sysctls themselves

This effectively removes these two strings from appearing as not
implemented when the check-sysctl-docs script is run

Signed-off-by: Joel Granados <joel.granados@kernel.org>