perf docs: Add man page entry for Arm SPE

The SPE integration in Perf has quite a few usability quirks that
can't be found by just reading the reference manual. So document this
and at the same time add a summary of the feature that is also hard to
find elsewhere.

Reviewed-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: James Clark <james.clark@arm.com>
Co-authored-by: Al Grant <al.grant@arm.com>
Co-authored-by: Luke Dare <Luke.Dare@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: German Gomez <german.gomez@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20220413084021.2556142-1-james.clark@arm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>