libperf evsel: Add comments for booleans

Add comments for 'system_wide' and 'requires_cpu' booleans
Signed-off-by: NAdrian Hunter <adrian.hunter@intel.com>
Acked-by: NIan Rogers <irogers@google.com>
Acked-by: NNamhyung Kim <namhyung@kernel.org>
Cc: Alexey Bayduraev <alexey.v.bayduraev@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Leo Yan <leo.yan@linaro.org>
Link: https://lore.kernel.org/r/20220524075436.29144-14-adrian.hunter@intel.comSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/perf/include/internal/evsel.h

@@ -49,7 +49,17 @@ struct perf_evsel {
 
 	/* parse modifier helper */
 	int			 nr_members;
+	/*
+	 * system_wide is for events that need to be on every CPU, irrespective
+	 * of user requested CPUs or threads. Map propagation will set cpus to
+	 * this event's own_cpus, whereby they will contribute to evlist
+	 * all_cpus.
+	 */
 	bool			 system_wide;
+	/*
+	 * Some events, for example uncore events, require a CPU.
+	 * i.e. it cannot be the 'any CPU' value of -1.
+	 */
 	bool			 requires_cpu;
 	int			 idx;
 };