perf pmus: Describe semantics of 'core_pmus' and 'other_pmus'

perf pmus: Describe semantics of 'core_pmus' and 'other_pmus'

Notion of 'core_pmus' and 'other_pmus' are independent of hw core and
uncore pmus. For example, AMD IBS PMUs are present in each SMT-thread
but they belongs to 'other_pmus'. Add a comment describing what these
list contains and how they are treated.

Reviewed-by: Ian Rogers <irogers@google.com>
Signed-off-by: Ravi Bangoria <ravi.bangoria@amd.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ali Saidi <alisaidi@amazon.com>
Cc: Ananth Narayan <ananth.narayan@amd.com>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sandipan Das <sandipan.das@amd.com>
Cc: Santosh Shukla <santosh.shukla@amd.com>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Link: https://lore.kernel.org/r/20230615051700.1833-2-ravi.bangoria@amd.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>