perf docs: arm_spe: Document new discard mode

Document the flag along with PMU events to hint what it's used for and
give an example with other useful options to get minimal output.

Reviewed-by: Yeoreum Yun <yeoreum.yun@arm.com>
Signed-off-by: James Clark <james.clark@linaro.org>
Link: https://lore.kernel.org/r/20250108142904.401139-3-james.clark@linaro.org


Signed-off-by: Will Deacon <will@kernel.org>