Document Transfer-Encoding behavior

[cpsievert]

As described here #550 (comment)

[cpsievert]

cc @meztez

[meztez]

Taken form https://github.com/rstudio/shiny/blob/a022a2b4a477a05682a140ff6b3ba6cfea823fba/R/middleware.R#L300

[schloerke]

Currently shiny does not even document this: https://github.com/rstudio/shiny/blob/64e09315fc5f38a31e1bf26179296a97be30b0be/R/shiny-options.R#L73-L74