chaingen: More precise ASERT comments.

This updates the comments detailing the origin of the polynomial
coefficients in the ASERT calculation to the more precise values which
also matches up with the standalone code.

blockchain/chaingen/generator.go

@@ -1308,9 +1308,9 @@ func calcASERTDiff(startDiffBits uint32, powLimit *big.Int, targetSecsPerBlock,
 	// guaranteed to be positive and a maximum of 17 bits, so it is safe to cast
 	// to a uint32.
 	const (
-		polyCoeff1 uint64 = 195766423245049 // 0.695502049 * 2^48
-		polyCoeff2 uint64 = 971821376       // 0.2262698 * 2^32
-		polyCoeff3 uint64 = 5127            // 0.0782318 * 2^16
+		polyCoeff1 uint64 = 195766423245049 // ceil(0.695502049712533 * 2^48)
+		polyCoeff2 uint64 = 971821376       // ceil(0.2262697964 * 2^32)
+		polyCoeff3 uint64 = 5127            // ceil(0.0782318 * 2^16)
 	)
 	fracFactor := uint32(1<<16 + (polyCoeff1*frac64+
 		polyCoeff2*frac64*frac64+