add kwargs documentation for DirectoryHandler (#7207)
add kwargs documentation for DirectoryHandler (#7207)

* add kwargs documentation for DirectoryHandler

* add documentation for argv kwarg in DirectoryHandler

ericmjl/bokeh,Karel-van-de-Plassche/bokeh,Karel-van-de-Plassche/bokeh,dennisobrien/bokeh,bokeh/bokeh,bokeh/bokeh,aavanian/bokeh,timsnyder/bokeh,jakirkham/bokeh,dennisobrien/bokeh,stonebig/bokeh,bokeh/bokeh,stonebig/bokeh,mindriot101/bokeh,timsnyder/bokeh,jakirkham/bokeh,Karel-van-de-Plassche/bokeh,dennisobrien/bokeh,timsnyder/bokeh,stonebig/bokeh,jakirkham/bokeh,timsnyder/bokeh,ericmjl/bokeh,mindriot101/bokeh,jakirkham/bokeh,mindriot101/bokeh,ericmjl/bokeh,ericmjl/bokeh,Karel-van-de-Plassche/bokeh,stonebig/bokeh,bokeh/bokeh,aavanian/bokeh,mindriot101/bokeh,aavanian/bokeh,bokeh/bokeh,aavanian/bokeh,dennisobrien/bokeh,timsnyder/bokeh,aavanian/bokeh,dennisobrien/bokeh,jakirkham/bokeh,Karel-van-de-Plassche/bokeh,ericmjl/bokeh