bug#18005: 24.4.50; In Emacs manual, tell users how to enter debugger for `user-error' errors

[Drew Adams]

The only hint about how to make Emacs enter the debugger for errors
deemed `user-error' is this mention, buried in the
`debug-ignored-errors' entry of the Elisp manual:

     The normal value of this variable includes `user-error', as well
     as several errors that happen often during editing but rarely
     result from bugs in Lisp programs.  However, "rarely" is not
     "never"; if your program fails with an error that matches this
     list, you may try changing this list to debug the error.  The
     easiest way is usually to set `debug-ignored-errors' to `nil'.

(FWIW, wrt this text: Is it really necessary to point out that
users can want to enter the debugger for other reasons that
tracking down a Lisp bug?)

It should be made *obvious* to users, in the *Emacs* manual, how to make
the debugger open for so-called "user errors".

In particular, some mention of how to do this should be added to any
descriptions of `debug-on-error' and `toggle-debug-on-error'.  Yes,
*especially* since these have NO effect on this.  Users will try these
and not understand how to get what they want in this regard.  Here
is one proof: http://superuser.com/q/782313/250462.

(Hard to believe that something presumably designed with end users in
mind would not be documented wrt how to access the debugger.  Of
course, `user-error' has NOTHING to do with user errors anyway...)

In GNU Emacs 24.4.50.1 (i686-pc-mingw32)
 of 2014-06-28 on ODIEONE
Bzr revision: 117431 rgm@gnu.org-20140628015517-eku6hj8mpgcvfnso
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/Devel/emacs/snapshot/trunk
 --enable-checking=yes,glyphs 'CFLAGS=-O0 -g3'
 LDFLAGS=-Lc:/Devel/emacs/lib 'CPPFLAGS=-DGC_MCHECK=1
 -Ic:/Devel/emacs/include''