perf Documentation: Clarify sysfs event names characters

Specify that perf event names in sysfs must not contain mixed lower and
upper case characters and that they may contain numbers, ".", "_",
or "-" as well.

Fixes: 785623ee ("perf Document: Sysfs event names must be lower or upper case")
Reviewed-by: Ian Rogers <irogers@google.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Link: https://lore.kernel.org/r/20240513192439.18473-1-rdunlap@infradead.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>