bug#5105: 23.1; doc string of facemenu-set-face

[Drew Adams]

> > What?  Why are you arbitrarily deciding that?  Why not assume that
> > allowing a string is a good thing, a purposeful design decision?
> 
> Because a good programmer knows that it's obviously not a good idea to
> encourage the caller to use a string here.

It might be obvious to what you call good programmers, but how about giving a
_reason_, for us mere mortals?

And what about all of the other places where we accept a string in place of the
object named by the string?  As I said:

>> We have many places in Emacs where we allow an arg to be
>> either a thing or its name.  Think of all of the BUFFER args
>> that can be a buffer or a buffer name.

What do the "good programmers" say about the design in those cases?  Is it
"obviously not a good idea" also?