[ODFF] Improvement for ACOSH

Hi members,

the semantics of 6.15.3 ACOSH seems odd to me. The formula has a 
plus-minus sign. That would mean, that it returns two values. But the 
function returns only the plus case.

The term "hyperbolic arc cosine" is not common usage, but "inverse 
hyperbolic cosine"; similar for ASINH and ATANH.

kind regards
Regina