add note about extension defaults (#11042)
add note about extension defaults (#11042)

* add note about extension defaults

* Apply suggestions from code review

Co-authored-by: Timo Cornelius Metzger <c920ee650506ce38c0734c98dde21c189c0817ef@gmx.net>

* more review comments

Co-authored-by: Timo Cornelius Metzger <c920ee650506ce38c0734c98dde21c189c0817ef@gmx.net>
bokeh/bokeh,bokeh/bokeh,bokeh/bokeh,bokeh/bokeh,bokeh/bokeh