Update documentation on text input.

[nelsonkam]

Is the website's documentation on text input handling up-to-date ? With @mpcomplete #6569 pull request, text input implementation seems unclear...

[sethladd]

Can someone summarize the issue? Or, better yet, send us a PR? :)

[sethladd]

See also #8725 (comment)

[sethladd]

I'm collecting places where we do document text input, in some fashion. Here's what I found thus far:

• https://docs.flutter.io/flutter/services/TextInput-class.html
• https://docs.flutter.io/flutter/material/Input-class.html
• https://docs.flutter.io/flutter/material/TextField-class.html
• https://docs.flutter.io/flutter/widgets/InputValue-class.html
• https://docs.flutter.io/flutter/material/InputContainer-class.html
• https://docs.flutter.io/flutter/services/TextInputClient-class.html
• https://flutter.io/text-input/

A vision:

• A single library for basic text input, which can get all the doc love and tell a holistic story, with an example.
• A widget for "materialized text input" in the material library
• A widget for "cupertinoized text input" in the cupertino library

If we determine we need more docs, beyond the "input library-level docs", we can update a HOWTO (simple cookbook style input solution).

[sethladd]

Related issue: #7575