Make `ComputedNode` documentation clearer (#24388)

# Objective

The ComputedNode docs are a bit confusing to a new reader. Add more
information and streamline them.

## Solution

- Linked back to `Node` for context.
- Explained when exactly the values are calculated.
- More consistency about the units used (physical pixels).
- Resolved meaning inversion at "For example" (where it first said don't
modify the fields, then gave an example of the opposite)
- Changed `0.` to "zero" because it reads easier (especially with the
punctuation)
- Removed mentions of `Auto` for values that are not `Val`.
- Made more specific about when change detection is bypassed. (The
original wording sounded like the user's updates to this field would
also bypass it)

## Testing

- Did you test these changes? If so, how?

`cargo doc -p bevy_ui` and looked at the result