The current docs for InputFormField aren't very helpful. We should probably update the docs for InputFormField, Input, and RawInput to all cross reference each other and explain when you might choose to use each one.

Also, https://flutter.io/text-input/ is out of date and has broken links. We should probably remove that page in favor of more details dartdocs. (We can link to the dartdocs from the web site.)

/cc @mpcomplete @Hixie