add comment

runtime/Java/src/org/antlr/v4/runtime/misc/IntegerList.java

@@ -272,6 +272,11 @@ public class IntegerList {
 		_data = Arrays.copyOf(_data, newLength);
 	}
 
+	/** Convert the list to a UTF-16 encoded char array. If all values are less
+	 *  than the 0xFFFF 16-bit code point limit then this is just a char array
+	 *  of 16-bit char as usual. For values in the supplementary range, encode
+	 * them as two UTF-16 code units.
+	 */
 	public final char[] toCharArray() {
 		// Optimize for the common case (all data values are
 		// < 0xFFFF) to avoid an extra scan