Add documentation to public constants

src/main/java/org/sqids/Sqids.java

@@ -17,8 +17,21 @@
  * This implementation is immutable and thread-safe, no lock is necessary.
  */
 public class Sqids {
+    /**
+    * The minimum allowable length of the alphabet used for encoding and
+    * decoding Sqids.
+    */
     public static final int MIN_ALPHABET_LENGTH = 3;
+
+    /**
+     * The maximum allowable minimum length of an encoded Sqid.
+     */
     public static final int MIN_LENGTH_LIMIT = 255;
+
+    /**
+     * The minimum length of blocked words in the block list. Any words shorter
+     * than the minimum are ignored.
+     */
     public static final int MIN_BLOCK_LIST_WORD_LENGTH = 3;
 
     private final String alphabet;