enh(Java): add comments for binary search (#776)

Improved the code by adding the explanation of using  (l + (r - l)) rather than using (l - r) while searching for the mid element.

algorithms/Java/searching/binary-search.java

@@ -6,6 +6,9 @@ class BinarySearch {
     { 
         if (r >= l) { 
             int mid = l + (r - l) / 2;  
+            //We use (l + (r - l)) rather than using (l - r) to avoid arithmetic overflow. 
+            //Arithmetic overflow is the situation when the value of a variable increases
+            //beyond the maximum value of the memory location, and wraps around.
   
             // If the element is present at the 
             // middle itself