Update javadocs

src/main/java/com/yahoo/sketches/quantiles/QuantilesSketchBuilder.java

@@ -37,10 +37,11 @@ public QuantilesSketchBuilder() {
 
   /**
    * Sets the parameter <i>k</i> that determines the accuracy and size of the sketch
-   * @param k determines the accuracy and size of the sketch.  <i>k</i> must be greater than 0 and
-   * less than 65536.
+   * @param k determines the accuracy and size of the sketch.  
+   * <i>k</i> must be greater than 0 and less than 65536.
    * It is recommended that <i>k</i> be a power of 2 to enable merging of sketches with
-   * different values of <i>k</i>.
+   * different values of <i>k</i>. However, in this case it is only possible to merge from 
+   * larger values of <i>k</i> to smaller values.
    * @return this builder
    */
   public QuantilesSketchBuilder setK(int k) {
@@ -115,7 +116,11 @@ public QuantilesSketch build() {
    * Returns a QuantilesSketch with the current configuration of this Builder and the
    * given parameter <i>k</i>.
    * @param k determines the accuracy and size of the sketch.  
-   * Must be greater than 0 and less than 65536.
+   * <i>k</i> must be greater than 0 and less than 65536. 
+   * It is recommended that <i>k</i> be a power of 2 to enable merging of sketches with
+   * different values of <i>k</i>. However, in this case it is only possible to merge from 
+   * larger values of <i>k</i> to smaller values.
+   * 
    * @return a QuantilesSketch
    */
   public QuantilesSketch build(int k) {

src/test/java/com/yahoo/sketches/quantiles/HeapQuantilesSketchTest.java

@@ -702,6 +702,14 @@ public void checkGetSeed() {
     assertEquals(qs1.getSeed(), 0);
   }
 
+  @Test(expectedExceptions = IllegalArgumentException.class)
+  public void checkImproperBuild() {
+    Memory mem = new NativeMemory(new byte[1024]);
+    @SuppressWarnings("unused")
+    QuantilesSketch qs1 = QuantilesSketch.builder().initMemory(mem).build(4);
+
+  }
+
   @Test
   public void checkKisOne() {
     QuantilesSketch qs1 = QuantilesSketch.builder().build(1);