Expand `--session-mode` documentation

[wurli]

Hi,

With the default --session-mode console errors come through on IoPub like this:

ename = "",
evalue = "Error in `dplyr::select()`:\n! Can't select columns that don't exist.",
traceback = { 
  " ▆",
  "1. ├─dplyr::select(dplyr::starwars, bla)",
  "2. ├─dplyr:::select.data.frame(dplyr::starwars, bla)",
  // More lines
}

But with --session-mode notebook you get the evalue prepended to the traceback:

ename = "",
evalue = "Error in `dplyr::select()`:\n! Can't select columns that don't exist.",
traceback = { 
  "Error in `dplyr::select()`:\n! Can't select columns that don't exist.",
  " ▆",
  "1. ├─dplyr::select(dplyr::starwars, bla)",
  "2. ├─dplyr:::select.data.frame(dplyr::starwars, bla)",
  // More lines
}

This follows the format expected by JupyterLab, which doesn't display the ename or evalue if the traceback is supplied.

Thankfully ark --install writes --session-mode notebook into the kernelspec, but I still managed to trip up on this and it took me a while to figure out what I was doing wrong!

Many thanks :)