Input docs and lifecycle improvements (#6321)
Input docs and lifecycle improvements (#6321)

I added some documentation to Input as I was learning how it worked
(and some minor tweaks, like handling reparenting better).
turnidge/flutter,turnidge/flutter,Hixie/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,tvolkert/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,flutter/flutter,tvolkert/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,turnidge/flutter,flutter/flutter,turnidge/flutter,flutter/flutter,cbracken/flutter,cbracken/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,Hixie/flutter,Hixie/flutter,Hixie/flutter,turnidge/flutter,cbracken/flutter,cbracken/flutter,tvolkert/flutter,cbracken/flutter,tvolkert/flutter