Fix MultiChildLayoutDelegate.hasChild doc

[TahaTesser]

fixes #115898

hasChild checks _idToChild map. _idToChild is not assigned until _callPerformLayout is called.
So hasChild shouldn't be called in getSize.

flutter/packages/flutter/lib/src/rendering/custom_layout.dart

Lines 400 to 404 in df789c9

	@override
	void performLayout() {
	size = _getSize(constraints);
	delegate._callPerformLayout(size, firstChild);
	}

Updated docs and example class names.

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[goderbauer]

"Avoid" is probably to weak here, this crashes if you call it from getSize, right? Maybe better:

"This method cannot be called from [getSize] as the size is not allowed to depend on the children" or something like that.

[goderbauer]

LGTM