Update documentation for functions that can return null

IPython/html/static/notebook/js/notebook.js

@@ -430,7 +430,7 b' define(['
      * 
      * @method get_cell
      * @param {Number} index An index of a cell to retrieve
-     * @return {Cell} A particular cell
+     * @return {Cell} Cell or null if no cell was found.
      */
     Notebook.prototype.get_cell = function (index) {
         var result = null;
@@ -446,7 +446,7 b' define(['
      * 
      * @method get_next_cell
      * @param {Cell} cell The provided cell
-     * @return {Cell} The next cell
+     * @return {Cell} the next cell or null if no cell was found.
      */
     Notebook.prototype.get_next_cell = function (cell) {
         var result = null;
@@ -462,7 +462,7 b' define(['
      * 
      * @method get_prev_cell
      * @param {Cell} cell The provided cell
-     * @return {Cell} The previous cell
+     * @return {Cell} The previous cell or null if no cell was found.
      */
     Notebook.prototype.get_prev_cell = function (cell) {
         var result = null;
@@ -478,7 +478,7 b' define(['
      * 
      * @method find_cell_index
      * @param {Cell} cell The provided cell
-     * @return {Number} The cell's numeric index
+     * @return {Number} The cell's numeric index or null if no cell was found.
      */
     Notebook.prototype.find_cell_index = function (cell) {
         var result = null;