Clarify what AnimatedBuilder really speeds up when using child

[fzyzcjy]

When I was reading AnimatedBuilder doc (when not that familiar w/ Flutter), I wrongly have the following conclusion: Suppose I have the following, and MyWidget constructs a huge tree:

return AnimatedBuilder(
  builder: () => AnimatedOpacity(child: Padding(padding: ..., child: MyWidget())),
);

vs

return AnimatedBuilder(
  builder: (child) => AnimatedOpacity(child: child),
  child: Padding(padding: ..., child: MyWidget()),
);

Then, reading the doc, I think this will be very little performance boost when using this child. It is because,

If your builder function contains a subtree that does not depend on the animation, it's more efficient to build that subtree once instead of rebuilding it on every animation tick.

So I thought: Yes, I do have a subtree (Padding + MyWidget) that does not change, and this trick can make them built once instead of many times. But it is very fast, because it just creates two objects! (Here, I understood the "a subtree" in the doc as the Padding+MyWidget two objects, while in reality we know it should mean the whole subtree that MyWidget and its descedent widgets build.)

Thus, I add a few lines to clarify this. Hope future flutter new learners do not get confused as me!

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[goderbauer]

This is basically repeating the information from the previous three paragraphs without really adding anything new.

[fzyzcjy]

Then maybe I should rephrase the paragraphs above. As is said in the post content of the PR, the original paragraphs can make new learners somehow confused and have wrong thoughts

[goderbauer]

I am going to close this as it doesn't add any new information to the doc.