[Cfp-interest 1455] proposed note about C math functions not required to be correctly rounded

[David Hough CFP]

Add a note for the second table in F.3:

IEC 60559 specifies correct rounding for many of the operations in this table, 
and thereby preserves useful mathematical properties like symmetry, 
monotonicity, and periodicity as much as possible.    
The corresponding cr_ functions in C do the same.
The C functions in this table, however, are not required to be
correctly rounded, but implementations should still preserve as many
useful mathematical properties as possible.