Explain that `runApp` forces the app to fill the screen (#5904)
Explain that `runApp` forces the app to fill the screen (#5904)

Fixes #5872
Hixie/flutter,tvolkert/flutter,flutter/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,turnidge/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,tvolkert/flutter,cbracken/flutter,cbracken/flutter,tvolkert/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,Hixie/flutter,Hixie/flutter,Hixie/flutter,turnidge/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,cbracken/flutter,turnidge/flutter,Hixie/flutter,turnidge/flutter,flutter/flutter,flutter/flutter,Hixie/flutter,flutter/flutter,turnidge/flutter,cbracken/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,tvolkert/flutter,cbracken/flutter