Bug 1542005 - Fix javadoc error in GeckoSession. r=geckoview-reviewers,rbarker

Differential Revision: https://phabricator.services.mozilla.com/D26219

--HG--
extra : moz-landing-system : lando

mobile/android/geckoview/src/main/java/org/mozilla/geckoview/GeckoSession.java

@@ -1793,7 +1793,7 @@ public class GeckoSession implements Parcelable {
          *
          * @param value The serialized SessionState in String form.
          * @return A new SessionState instance.
-         * @throws JSONException
+         * @throws JSONException if the value is not a valid json
          */
         public static @NonNull SessionState fromString(final @NonNull String value) throws JSONException {
             return new SessionState(GeckoBundle.fromJSONObject(new JSONObject(value)));