Better documentation for Transform and GlobalTransform

[alice-i-cecile]

What problem does this solve or what need does it fill?

Transforms are commonly used by almost all users of Bevy, including beginners. They have some unintuitive and undocumented behavior however, especially regarding Transform vs. GlobalTransform and how all this relates to parent-child object hierarachies.

What solution would you like?

Clearly document:

1. What Transform and GlobalTransform are.
2. Their basic usage.
3. Which one you should use in which cases.
4. How they interact with parents / children.
5. How and when the transform hierarchy is updated.
6. If [Merged by Bors] - Reliable change detection #1471 has not yet been merged, the perils of Changed in the transform hierarchy updater.

What alternative(s) have you considered?

Safer, clearer API. This is hard to do without losing performance, see #1460.

Additional context

Other approaches to solve this confusion was discussed at length in #1460, and better documentation was proposed as a good stopgap until a clear improvement is reached.