The rules are:
Permission to copy without fee all or part of this material is granted,
provided the OpenMP Architecture Review Board copyright notice and
the title of this document appear. Notice is given that copying is by
permission of OpenMP Architecture Review Board
So the text depends upon the version of the API that you are using. If the compiler is a OpenMP 3.1 compiler, the text would be:
OpenMP Application Program Interface
Version 3.1 July 2011
Copyright © 1997-2011 OpenMP Architecture Review Board.
You are also allowed to use the OpenMP trademark in your documentation, provided you mention the following text:
"The OpenMP name is a registered trademark of the OpenMP Architecture Review Board."
and you also follow the OpenMP trademark by a registered trademark symbol (®), or mark, at its first use and first prominent use in a document.
Matthijs van Waveren