perf x86 evlist: Update comments on topdown regrouping

Update to remove comments about groupings not working and with the:
```
perf stat -e "{instructions,slots},{cycles,topdown-retiring}"
```
case that now works.

Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250307023906.1135613-4-irogers@google.com


Signed-off-by: Namhyung Kim <namhyung@kernel.org>