Add more warm up frame docs

[loic-sharma]

Updated the wiki to call out the warm up frame. This contains details from the following pages:

1. flutter/packages/flutter/lib/src/scheduler/binding.dart

   Lines 994 to 1016 in b98d485

   	/// ## Design discussion
   	///
   	/// The Flutter engine prompts the framework to generate frames when it
   	/// receives a request from the operating system (known for historical reasons
   	/// as a vsync). However, this may not happen for several milliseconds after
   	/// the app starts (or after a hot reload). To make use of the time between
   	/// when the widget tree is first configured and when the engine requests an
   	/// update, the framework schedules a _warm-up frame_.
   	///
   	/// A warm-up frame may never actually render (as the engine did not request
   	/// it and therefore does not have a valid context in which to paint), but it
   	/// will cause the framework to go through the steps of building, laying out,
   	/// and painting, which can together take several milliseconds. Thus, when the
   	/// engine requests a real frame, much of the work will already have been
   	/// completed, and the framework can generate the frame with minimal
   	/// additional effort.
   	///
   	/// Warm-up frames are scheduled by [runApp] on startup, and by
   	/// [RendererBinding.performReassemble] during a hot reload.
   	///
   	/// Warm-up frames are also scheduled when the framework is unblocked by a
   	/// call to [RendererBinding.allowFirstFrame] (corresponding to a call to
   	/// [RendererBinding.deferFirstFrame] that blocked the rendering).

2. https://api.flutter.dev/flutter/dart-ui/PlatformDispatcher/scheduleWarmUpFrame.html
3. https://api.flutter.dev/flutter/dart-ui/FlutterView/physicalSize.html

Part of #149974

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.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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[goderbauer]

LGTM

[dkwingsmt]

Suggested change

	A warm-up frame may never actually render (as the engine did not request
	it and therefore does not have a valid context in which to paint), but it
	Depending on the platform implementation, the warm-up frame might cause the first frame to appear slightly earlier
	than without, or might not be presented at all. In either case, it

[dkwingsmt]

I think the "engine context" is not important. And I suggest clarifying the side effect of the first frame time (if I understand the mechanism correctly)

[loic-sharma]

Hm maybe instead of "engine context" I'll tweak this to say something like:

... as it invokes FlutterView.render outside of the scope of PlatformDispatcher.onBeginFrame or PlatformDispatcher.onDrawFrame

That phrasing would be more accurate while also giving some useful context. These are meant to be contributor-facing docs, so I think these kinds of details might be useful.

[dkwingsmt]

Suggested change

	and painting, which can together take several milliseconds. Thus, when the
	engine requests a real frame, much of the work will already have been
	completed, and the framework can generate the frame with minimal
	additional effort.
	and painting, which allows the first real frame to be generated accumulatively, shortening the delay.

Maybe a more concise way?

[loic-sharma]

I agree the current text is verbose, but I think it's clearer. Personally, I'd prefer to optimize for clarity here. (I stole this text from the framework's docs, not tooting my own horn here 😂)

[dkwingsmt]

Is this paragraph really needed? Is it something that really confuses people?

[dkwingsmt]

We can probably just count this case as another case of "the warm up frame is not presented at all".

[loic-sharma]

Hey there, I agree this paragraph is a bit deep in the weeds. But this topic is the reason why I sent this pull request, as a follow-up to answering this question: #149974

[dkwingsmt]

I see, thanks for explanation!

[dkwingsmt]

Thanks for the work! In general I think this is good change. I have some suggestion, but none of them are strong.

If possible, it might be worthwhile asking someone who's not familiar with this topic to see if the docs make enough sense.

[dkwingsmt]

We can probably just count this case as another case of "the warm up frame is not presented at all".