bug#9700: 24.0.50; doc string of `key-description'

[Chong Yidong]

"Drew Adams" <drew.adams@oracle.com> writes:

>  "Control characters turn into "C-foo" sequences, meta into "M-foo",
>   spaces are put between sequence elements, etc."
>  
> Since Emacs 21 that needs to be changed to "<C-foo>" and "<M-foo>",
> since "foo" is obviously the name of a function key (more than one
> character in the name).

No, these are not referring to function keys.  I tweaked the docstring
to make this clearer.