What to do with the debugger buffer when exiting `debug'.
The value affects the behavior of operations on any window
previously showing the debugger buffer.
`nil' means that if its window is not deleted when exiting the
debugger, invoking `switch-to-prev-buffer' will usually show
the debugger buffer again.
`append' means that if the window is not deleted, the debugger
buffer moves to the end of the window's previous buffers so
it's less likely that a future invocation of
`switch-to-prev-buffer' will switch to it. Also, it moves the
buffer to the end of the frame's buffer list.
`bury' means that if the window is not deleted, its buffer is
removed from the window's list of previous buffers. Also, it
moves the buffer to the end of the frame's buffer list. This
value provides the most reliable remedy to not have
`switch-to-prev-buffer' switch to the debugger buffer again
without killing the buffer.
`kill' means to kill the debugger buffer. The value used here is passed to `quit-restore-window'.