perf Document: Sysfs event names must be lower or upper case

To avoid directory scans in perf it is going to be assumed that sysfs
event names are either lower or upper case.

Reviewed-by: Kan Liang <[e-mail]>
Signed-off-by: Ian Rogers <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Bjorn Helgaas <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: James Clark <[e-mail]>
Cc: Jing Zhang <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Randy Dunlap <[e-mail]>
Cc: Ravi Bangoria <[e-mail]>
Cc: Thomas Richter <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

Documentation/ABI/testing/sysfs-bus-event_source-devices-events

···
 		performance monitoring event supported by the <pmu>. The name
 		of the file is the name of the event.
 
+		As performance monitoring event names are case
+		insensitive in the perf tool, the perf tool only looks
+		for lower or upper case event names in sysfs to avoid
+		scanning the directory. It is therefore required the
+		name of the event here is either lower or upper case.
+
 		File contents:
 
 			<term>[=<value>][,<term>[=<value>]]...