Clarifying that Dividers aren't restricted to usage in lists and Drawers. Maybe no one else is confused about this but I didn't realize it until I found them used in the calculator demo (https://github.com/flutter/flutter/blob/8ca4caa4406046a683c0e2d2a5cf6118fa56fef7/examples/flutter_gallery/lib/demo/calculator/home.dart)

...until people understand them.

Based on observations of usability study participant P1.

This patch completes the circuit on this library. The next step is to go
through and add docs to anything we missed.

Fixes https://github.com/flutter/flutter/issues/3363

The dartdoc will continue until morale improves.

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308

Fixes #1382