As for the the example I can tell you precisely why it's documented that way. "DMA engine randomly locks up" is a hardware bug that customers may demand fixed at great expense. A typical full layer spin for an IC will run you about $500,000 these days and that's if you have push a lot of volume. "Reserved" is no problem at all. Just don't do that. Simple.
I've coded a few magic constants in my day but they've been clearly labels as such with at least an attempt to describe what they do. Mostly they are escape hatches in case of bugs elsewhere... In any case my neck remain un-wrung to I must be doing something right.
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