docs: add info to SharedPreferences.Editor.putString

         * {@link #commit} or {@link #apply} are called.

         * 

         * @param key The name of the preference to modify.

         * @param value The new value for the preference.

         * @param value The new value for the preference.  Passing {@code null}

         *    for this argument is equivalent to calling {@link #remove(String)} with

         *    this key.

         * 

         * @return Returns a reference to the same Editor object, so you can

         * chain put calls together.