Document No Programmatic JS access from Notebook

This closes https://github.com/jupyterlab/jupyterlab/issues/4623
by documenting why we don't support writing JS code as notebook
outputs that modify the JupyterLab UI.

docs/source/getting_started/faq.rst

@@ -21,6 +21,16 @@ Development
 -  `How can you
    contribute? <https://github.com/jupyterlab/jupyterlab/blob/master/CONTRIBUTING.md>`__
 -  :ref:`How can you extend or customize JupyterLab? <user_extensions>`
+-  In the classic Notebook, `I could use custom Javascript outputed by a cell to programatically
+   control the Notebook <https://stackoverflow.com/a/32769976/907060>`__. Can I do the same thing in JupyterLab?
+
+   We intentially do not support this workflow. Instead, we reccomend you display a new mimetype and define a
+   custom renderer for that in a JupyterLab extension. This makes sure the output is not tied directly to JupyterLab
+   so that another frontend could still render it. Also, if the JupyterLab API changes then
+   this change can be supported in your extension and the existing notebooks will still work without changing their
+   outputs. If you have comments or suggestions on changes here, we have `an issue <https://github.com/jupyterlab/jupyterlab/issues/4623>`__
+   to discuss this decision.
+
 
 Tips and Tricks
 ---------------