improve trait object doc

[wiseaidev]

Related to #392

[traviscross]

@wiseaidev: We notice this is still a draft PR. Do you still plan to move this forward?

In skimming the draft, this seems to add a lot of examples. While we like examples, this is probably too many.

There's also some language that maybe tries a bit too hard to sell the language. E.g. "is a powerful concept", and similar.

We're also a bit unclear about where the confusion is that's motivating this work. Perhaps you could explain better here, in the PR description what the motivation is, and what the nature of the confusion is that you're trying to resolve with these changes.

[rustbot]

☔ The latest upstream changes (possibly bf115a4) made this pull request unmergeable. Please resolve the merge conflicts.