a warning to implement

[Jason Riedy]

And Gabriel Dos Reis writes:
 - 
 - It is defined to copy a signalling NaN.

Nit:  Signalling behavior on copying _with_no_change_in_format_
is up to the implementation in 754-1985.  The language suggests 
uniformity, so it should always signal invalid or never signal 
invalid.  The revision committee has agreed it should be quiet 
always in a future standard.  I don't know about format changes,
though.  I'll have to bring that up.  (Think loading a non-80-
bit-quantity in x86, which is already not 754-1985, but still...)

Jason