Update docs for benchmarks (#5999)
Update docs for benchmarks (#5999)

We changed how `flutter run --release` works (it quits without showing
logs now, since it can't tell when the app closes).
tvolkert/flutter,cbracken/flutter,Hixie/flutter,tvolkert/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,turnidge/flutter,flutter/flutter,turnidge/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,Hixie/flutter,Hixie/flutter,Hixie/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,turnidge/flutter,Hixie/flutter,tvolkert/flutter,turnidge/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,turnidge/flutter