Add more docs and examples for implementers of RenderSliver (#15419)
Add more docs and examples for implementers of RenderSliver (#15419)

* Add more docs and examples for implementers of RenderSliver

* add more typical scenarios to describe scrollOffset

* review

* tweak a word

cbracken/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,cbracken/flutter,flutter/flutter,Hixie/flutter,tvolkert/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,cbracken/flutter,flutter/flutter,tvolkert/flutter,cbracken/flutter,Hixie/flutter,tvolkert/flutter,cbracken/flutter