bug#15789: 24.3.50; backtrace produces unreadable output

[Jarek Czekalski <jarekczek@poczta.onet.pl>]

This is the code I use to produce an example backtrace:

(defadvice tmm-add-prompt (before emacspeak pre act comp)
   "Speaks the list of completions we have available."
   (backtrace))

Then launch M-x tmm-menubar to have it output the backtrace.

When I test it on Emacs 23.4, I have a nice backtrace output, with one
readable line per stack item. On r114884 this looks really ugly,
unreadable. Some bytecode lines are mixed with words.

The stack trace looks as though the compiled-function lisp type is
printed, so I suspect that specbinding.bt.function changed its type,
which also affects Vsignaling_function argument to
command-error-function. I propose documenting the Lisp type of this
structure member.

In GNU Emacs 24.3.50.4 (i686-pc-linux-gnu, GTK+ Version 3.8.4)
  of 2013-11-01 on jcdeb
Bzr revision: 114884 rgm@gnu.org-20131031213910-3509l9e973ne3zy1
Windowing system distributor `The X.Org Foundation', version 11.0.11204000
System Description:    Debian GNU/Linux testing (jessie)

Important settings:
   value of $LC_ALL: en_US
   value of $LANG: pl_PL
   locale-coding-system: iso-latin-1-unix
   default enable-multibyte-characters: t

Major mode: Messages

Minor modes in effect:
   tooltip-mode: t
   mouse-wheel-mode: t
   tool-bar-mode: t
   menu-bar-mode: t
   file-name-shadow-mode: t
   global-font-lock-mode: t
   font-lock-mode: t
   blink-cursor-mode: t
   auto-composition-mode: t
   auto-encryption-mode: t
   auto-compression-mode: t
   buffer-read-only: t
   line-number-mode: t
   transient-mark-mode: t

Recent input:
C-x C-f / j a r e k / l a n g / g n u / e m a c s p
e / t <backspace> <backspace> <tab> t m m <tab> <return>
M-x e v a l - b <tab> <return> M-` <escape> <escape>
<escape> C-x C-b <down> <f6> C-x o <down> <down> <return>
<prior> <next> <prior> <prior> <prior> <f11> <f11>
<up> <up> <up> M-x t o g g l e - t r u <tab> <return>
<home> <next> <prior> <prior> <prior> M-x r e p o r
t <tab> <return>

Recent messages:
[very long, removed intentionally by the reporter]

Load-path shadows:
None found.

Features:
(shadow sort gnus-util mail-extr emacsbug message format-spec rfc822 mml
mml-sec mm-decode mm-bodies mm-encode mail-parse rfc2231 mailabbrev
gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums mm-util
mail-prsvr mail-utils help-mode easymenu tmm electric advice help-fns
time-date tooltip ediff-hook vc-hooks lisp-float-type mwheel x-win x-dnd
tool-bar dnd fontset image regexp-opt fringe tabulated-list newcomment
lisp-mode prog-mode register page menu-bar rfn-eshadow timer select
scroll-bar mouse jit-lock font-lock syntax facemenu font-core frame cham
georgian utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao
korean japanese hebrew greek romanian slovak czech european ethiopic
indian cyrillic chinese case-table epa-hook jka-cmpr-hook help simple
abbrev minibuffer nadvice loaddefs button faces cus-face macroexp files
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote make-network-process
dbusbind gfilenotify dynamic-setting system-font-setting
font-render-setting move-toolbar gtk x-toolkit x multi-tty emacs)