LUCENE-3662: add note

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1224823 13f79535-47bb-0310-9956-ffa450edef68

modules/suggest/src/java/org/apache/lucene/search/spell/LuceneLevenshteinDistance.java

@@ -45,6 +45,12 @@ public float getDistance(String target, String other) {
     IntsRef otherPoints;
     int n;
     int d[][]; // cost array
+
+    // NOTE: if we cared, we could 3*m space instead of m*n space, similar to 
+    // what LevenshteinDistance does, except cycling thru a ring of three 
+    // horizontal cost arrays... but this comparator is never actually used by 
+    // DirectSpellChecker, its only used for merging results from multiple shards 
+    // in "distributed spellcheck", and its inefficient in other ways too...
 
     // cheaper to do this up front once
     targetPoints = toIntsRef(target);