"frame times may be negatively affected" is not quite the right message

We have these checkboxes that add more instrumentation:
https://github.com/flutter/devtools/blob/master/packages/devtools_app/lib/src/performance/performance_screen.dart#L389

We have a message that says that "frame times may be negatively affected" when these are on, which is true. However, I'm not sure that fully represents the issue with these flags. People might see that and think, ok, I'll ignore the _numbers_, but the _relative_ sizes of work loads is still a guide as to what I should work on... which is wrong. It's possible for a widget to take 5ms with these flags off, and be the main problem that causes jank, but for these flags to cause other widgets to take 5ms as well while the problematic widget continues to only take 5ms, and now the source of the jank has been obscured.

The problem is that the frame times can be affected in _non-representative ways_.

Anyway I tried to submit a PR to fix this but I could not come up with wording that was accurate and comprehensible. So I'm filing this bug instead.

cc @InMatrix