Improve docs for keepExisting parameter
Improve docs for keepExisting parameter

blink1073/jupyter-js-filebrowser,jupyter/jupyter-js-filebrowser,jupyter/jupyter-js-filebrowser,blink1073/jupyter-js-filebrowser,blink1073/jupyter-js-filebrowser,blink1073/jupyter-js-filebrowser,blink1073/jupyter-js-filebrowser,jupyter/jupyter-js-filebrowser,jupyter/jupyter-js-filebrowser,jupyter/jupyter-js-filebrowser