Clarify that updates after commit() is called but before promise resolution are sync

[chrishtr]

Consider the following code:

element.displayLock.acquire(() => {
element.displayLock.commit();
element.offsetLeft;
}

In this situation, offsetLeft left would use the committed offsetLeft of the layout of the element. OTOH:

element.displayLock.acquire(() => {
element.offsetLeft;
}

Here offsetLeft would only be based on the intrinsic sizing of the element, not its surrounding context.

[vmpstr]

deprecated by rendersubtree