[stdlib] Expand integer conversion comment

stdlib/public/core/Integers.swift

@@ -3010,7 +3010,10 @@ extension FixedWidthInteger {
     let minBitWidth = source.significandWidth
     let isExact = (minBitWidth <= exponent)
     let bitPattern = source.significandBitPattern
-    // Determine the number of meaningful bits in the significand bit pattern.
+    // Determine the actual number of fractional significand bits.
+    // `Source.significandBitCount` would not reflect the actual number of
+    // fractional significand bits if `Source` is not a fixed-width floating-point
+    // type; we can compute this value as follows if `source` is finite:
     let bitWidth = minBitWidth &+ bitPattern.trailingZeroBitCount
     let shift = exponent - Source.Exponent(bitWidth)
     // Use `Self.Magnitude` to prevent sign extension if `shift < 0`.