One way to find the specific reference to TWOV is to query Timatic using PEK as the transit point. In your case, query US-SIN. Then look under the transit provision (China). The TWOV exceptions will be shown.
I expect.
I say that because an AAgent at SFO showed me how to find the same transit provision for my upcoming US-PVG-NRT-US trip. It didn't come up when you query US-PVG/PEK, only when you query US-SIN/NRT and look for transit via PEK/PVG/et al.