@dfn{foo} should generate “foo” in UTF-8

[Paul Eggert]

The GNU Emacs manuals use ‘@documentencoding UTF-8’, so input ``like this'' 
generates info files with curved double quotes “like this”.  However, input 
@dfn{like this} generates info files "like this", with ASCII double-quotes.  Can 
you please fix @dfn so that it generates curved instead of ASCII double quotes 
when the rest of the output uses curved quotes?  Thanks.