Improve comment in BloomFilterIndexer.

src/main/java/org/apache/commons/collections4/bloomfilter/BloomFilterIndexer.java

@@ -53,7 +53,9 @@ final class BloomFilterIndexer {
      * @see #checkPositive(int)
      */
     static int getLongIndex(int bitIndex) {
-        // Use a signed shift. Any negative index will produce a negative value
+        // An integer divide by 64 is equivalent to a shift of 6 bits if the integer is positive.
+        // We do not explicitly check for a negative here. Instead we use a
+        // a signed shift. Any negative index will produce a negative value
         // by sign-extension and if used as an index into an array it will throw an exception.
         return bitIndex >> DIVIDE_BY_64;
     }