`render_cell()` hook should take an optional `request` argument

[simonw]

From Discord: https://discordapp.com/channels/823971286308356157/996877076982415491/1068227071156965486

[simonw]

Updated documentation: https://docs.datasette.io/en/latest/plugin_hooks.html#render-cell-row-value-column-table-database-datasette-request