bug#8789: 23.3; debug backtrace buffer changes window on step-through

[Drew Adams]

> Try customizing `debugger-bury-or-kill'.

1. OK, I was not aware of that option; thanks.  Please document it.  (I do not
see it in either the Emacs manual or the Elisp manual.)

2. I tried each of the values listed in the doc string.  None of them had any
effect at all.  Same broken behavior, always: the frame is deleted, then
re-created with the default size and position (i.e., ignoring its previous size
and position).  That re-creation takes perhaps 0.5 sec or 1 sec, which means
that it is visually quite distracting, besides the fact that the size and
position prevent using the debugger efffectively (really, prevent using it at
all).

I am using the regular Emacs debugger, not `edebug'.  E.g., I am using
`debug-on-entry' etc.  Dunno whether that is pertinent.  The doc says nothing
about `edebug', in any case.

3. Wrt the new option's defcustom: If the documented four possible values are
the only possible values, then the defcustom should be rewritten so that the
user can choose (and can only choose) by using the Value Menu.  S?he should not
have to or be allowed to enter a quoted symbol name in a text edit field.

If the documented possible values are not the only possible values, then the doc
string should be updated to mention what other possible values there are and
what they do.

This (#3) is a bug.  Do you want me to file a separate bug report for it?