[docs] render.DataGrid and render.DataTable should mention selected rows

[gadenbuie]

render.data_frame mentions the {id}_selected_rows() input here https://shiny.posit.co/py/api/render.data_frame.html#row-selection, but neither render.DataGrid now render.DataTable mention this information, making it hard to discover.

Discoverability might be improved by #975, but the documentation for render.DataGrid and render.DataTable should mention how to access selected rows.

[SamEdwardes]

+1 I came across this today. I think this section of the docs (https://shiny.posit.co/py/api/core/render.data_frame.html#row-selection) should be updated to use the new API (e.g. .data_view(selected=True))