Rewrite comments on `example/tree.dart`

[chykon]

Motivation

The comment style in the example/tree.dart example is different from the other examples. Subjectively, it is more difficult for beginners.

Desired solution

Rewrite the comments, linking them to specific code snippets, as done in other examples.

Alternatives considered

Ignore it.

Additional details

No response

[chykon]

Perhaps double slashes should be used instead of triple slashes for comments since there is no need for documentation?

[mkorbel1]

Good points, thank you for filing!

[chykon]

There is also a doc/TreeExample.md file, the content of which is almost the same as the example/tree.dart example. It may not be needed after the example/tree.dart example has been reworked, or it could be included in tutorials to demonstrate generation capabilities.