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

[Drew Adams]

> The whole point of introducing user-error was so that uninteresting
> errors will not trigger the debugger. I see no need to mention in the
> *Emacs* manual how to get round this.

"The whole point" should be to _let_ users not be bothered entering
the debugger for uninteresting errors.  That's reasonable behavior.
*Enabling* that behavior is positive.  Forcing it would be
counterproductive, as is not making clear what it is that controls it.

Not to mention the fact that what constitutes an "uninteresting
error" is not something fixed once and for all for everyone.  It
can change, depending on the context.  A user might well want to
take a look under the covers using the debugger in some context.
(The user who just now asked how to do it, for instance.)

> (I am not going to get into a big debate about this.)

No one asked you to.