Add clarification comment in SparseFileContent.Range.toString

Change I34fb07f incorrectly "fixed" the brackets in the `toString`
method, however the usage of [ and ) was intentional, to denote that
the range is inclusive/exclusive.

Add a short comment in the method to prevent the same mistake from
being made again.

Change-Id: Idfe9b560c0cc55bfa4a6ab06fba172d971c3089c

gerrit-prettify/src/main/java/com/google/gerrit/prettify/common/SparseFileContent.java

@@ -296,6 +296,7 @@ public class SparseFileContent {
 
     @Override
     public String toString() {
+      // Usage of [ and ) is intentional to denote inclusive/exclusive range
       return "Range[" + base + "," + end() + ")";
     }
   }