Document why rebuild indicators are missing for some widgets (Column, ...)

[InMatrix]

Steps to Reproduce

Run the perf diagnosis demo app and turn on the rebuild indicators. In clock_demo.dart, I'd expect to see an indicator for the Column widget's constructor (Line 37).

[jacob314]

This is actually a package:flutter issue. We are technically recording the correct rebuilds. The rebuild method isn't being called on the column for reasons related to RenderObjectElement updating the associated render object instead of doing a rebuild.

[devoncarew]

@jacob314, is there work for us to do here? Perhaps this should be mentioned in documentation for the rebuild feature?

[InMatrix]

From a discussion with @Hixie in the inspector chat room. the reason why Column didn't get a rebuild indicator was because it's a RenderObjectWidget. This hasn't caused confusion in our user study, but it might undermine the perceived credibility of the information shown to the user if noticed frequently. We should at least mention that in the documentation of the feature.