Make ComputedNode documentation clearer

[amtep]

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