improve the helpstring for CDS selected property (#5816)
improve the helpstring for CDS selected property (#5816)

fixes #5718
mindriot101/bokeh,philippjfr/bokeh,aiguofer/bokeh,dennisobrien/bokeh,Karel-van-de-Plassche/bokeh,bokeh/bokeh,timsnyder/bokeh,aavanian/bokeh,mindriot101/bokeh,timsnyder/bokeh,ericmjl/bokeh,DuCorey/bokeh,jakirkham/bokeh,percyfal/bokeh,philippjfr/bokeh,ericmjl/bokeh,aiguofer/bokeh,jakirkham/bokeh,aiguofer/bokeh,dennisobrien/bokeh,Karel-van-de-Plassche/bokeh,percyfal/bokeh,schoolie/bokeh,timsnyder/bokeh,timsnyder/bokeh,rs2/bokeh,rs2/bokeh,aavanian/bokeh,schoolie/bokeh,jakirkham/bokeh,DuCorey/bokeh,bokeh/bokeh,schoolie/bokeh,percyfal/bokeh,aiguofer/bokeh,mindriot101/bokeh,percyfal/bokeh,jakirkham/bokeh,ericmjl/bokeh,Karel-van-de-Plassche/bokeh,mindriot101/bokeh,percyfal/bokeh,jakirkham/bokeh,rs2/bokeh,DuCorey/bokeh,stonebig/bokeh,bokeh/bokeh,ericmjl/bokeh,ericmjl/bokeh,Karel-van-de-Plassche/bokeh,Karel-van-de-Plassche/bokeh,dennisobrien/bokeh,aavanian/bokeh,schoolie/bokeh,DuCorey/bokeh,bokeh/bokeh,schoolie/bokeh,timsnyder/bokeh,philippjfr/bokeh,aavanian/bokeh,rs2/bokeh,rs2/bokeh,aavanian/bokeh,dennisobrien/bokeh,aiguofer/bokeh,stonebig/bokeh,stonebig/bokeh,bokeh/bokeh,philippjfr/bokeh,DuCorey/bokeh,stonebig/bokeh,philippjfr/bokeh,dennisobrien/bokeh