Fix javadoc warnings

[niloc132]

Not required for rc release.

19 warnings
[WARNING] Javadoc Warnings
[WARNING] /home/colin/workspace/gwt-json/gwt-json/src/main/java/org/gwtproject/json/client/JSONArray.java:60: warning: no @return
[WARNING] public JavaScriptObject getJavaScriptObject() {
[WARNING] ^
[WARNING] /home/colin/workspace/gwt-json/gwt-json/src/main/java/org/gwtproject/json/client/JSONArray.java:67: warning: empty <p> tag
[WARNING] * <p>
[WARNING] ^
[WARNING] /home/colin/workspace/gwt-json/gwt-json/src/main/java/org/gwtproject/json/client/JSONArray.java:69: warning: no @return
[WARNING] public JsArray<Object> getArray() {
[WARNING] ^
[WARNING] /home/colin/workspace/gwt-json/gwt-json/src/main/java/org/gwtproject/json/client/JSONBoolean.java:53: warning: no @return
[WARNING] public boolean booleanValue() {
[WARNING] ^
[WARNING] /home/colin/workspace/gwt-json/gwt-json/src/main/java/org/gwtproject/json/client/JSONException.java:27: warning: no @param for message
[WARNING] public JSONException(String message) {
[WARNING] ^
[WARNING] /home/colin/workspace/gwt-json/gwt-json/src/main/java/org/gwtproject/json/client/JSONException.java:32: warning: no @param for message
[WARNING] public JSONException(String message, Throwable cause) {
[WARNING] ^
[WARNING] /home/colin/workspace/gwt-json/gwt-json/src/main/java/org/gwtproject/json/client/JSONException.java:32: warning: no @param for cause
[WARNING] public JSONException(String message, Throwable cause) {
[WARNING] ^
[WARNING] /home/colin/workspace/gwt-json/gwt-json/src/main/java/org/gwtproject/json/client/JSONException.java:37: warning: no @param for cause
[WARNING] public JSONException(Throwable cause) {
[WARNING] ^
[WARNING] /home/colin/workspace/gwt-json/gwt-json/src/main/java/org/gwtproject/json/client/JSONNull.java:24: warning: no @return
[WARNING] public static JSONNull getInstance() {
[WARNING] ^
[WARNING] /home/colin/workspace/gwt-json/gwt-json/src/main/java/org/gwtproject/json/client/JSONNumber.java:29: warning: no @return
[WARNING] public double doubleValue() {
[WARNING] ^
[WARNING] /home/colin/workspace/gwt-json/gwt-json/src/main/java/org/gwtproject/json/client/JSONNumber.java:47: warning: no @return
[WARNING] public double getValue() {
[WARNING] ^
[WARNING] /home/colin/workspace/gwt-json/gwt-json/src/main/java/org/gwtproject/json/client/JSONNumber.java:24: warning: no @param for value
[WARNING] public JSONNumber(double value) {
[WARNING] ^
[WARNING] /home/colin/workspace/gwt-json/gwt-json/src/main/java/org/gwtproject/json/client/JSONObject.java:57: warning: no @return
[WARNING] public JavaScriptObject getJavaScriptObject() {
[WARNING] ^
[WARNING] /home/colin/workspace/gwt-json/gwt-json/src/main/java/org/gwtproject/json/client/JSONObject.java:111: warning: no @return
[WARNING] public Set<String> keySet() {
[WARNING] ^
[WARNING] /home/colin/workspace/gwt-json/gwt-json/src/main/java/org/gwtproject/json/client/JSONObject.java:150: warning: no @return
[WARNING] public int size() {
[WARNING] ^
[WARNING] /home/colin/workspace/gwt-json/gwt-json/src/main/java/org/gwtproject/json/client/JSONObject.java:46: warning: no @param for jsValue
[WARNING] public JSONObject(JavaScriptObject jsValue) {
[WARNING] ^
[WARNING] /home/colin/workspace/gwt-json/gwt-json/src/main/java/org/gwtproject/json/client/JSONObject.java:51: warning: no @param for jsValue
[WARNING] public JSONObject(JsObject jsValue) {
[WARNING] ^
[WARNING] /home/colin/workspace/gwt-json/gwt-json/src/main/java/org/gwtproject/json/client/JSONParser.java:32: warning: empty <p> tag
[WARNING] * <p>
[WARNING] ^
[WARNING] /home/colin/workspace/gwt-json/gwt-json/src/main/java/org/gwtproject/json/client/JSONString.java:59: warning: no @return
[WARNING] public String stringValue() {
[WARNING] ^