"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

检查 packages/devtools_app/lib/src/performance/performance_screen.dart 中 instrumentation 复选框以及现有的“帧时间可能会受到负面影响”消息附近的内容。优化措辞，使用户了解 instrumentation 可能会以不具代表性的方式扭曲帧工作负载，包括相对比较，然后在性能界面中验证该消息。

由索引模型根据 Issue 内容生成。