Improve docs for MediaQueryData.size
Improve docs for MediaQueryData.size

This would have answered a question we go on flutter-dev.

Fixes #3517

flutter/flutter,cbracken/flutter,turnidge/flutter,aghassemi/flutter,turnidge/flutter,tvolkert/flutter,cbracken/flutter,cbracken/flutter,turnidge/flutter,tvolkert/flutter,aghassemi/flutter,cbracken/flutter,turnidge/flutter,cbracken/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,aghassemi/flutter,tvolkert/flutter,cbracken/flutter,tvolkert/flutter,Hixie/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,turnidge/flutter,flutter/flutter,Hixie/flutter,flutter/flutter,Hixie/flutter,aghassemi/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,flutter/flutter,flutter/flutter,tvolkert/flutter,flutter/flutter,Hixie/flutter,Hixie/flutter,cbracken/flutter