msun: document that {sin,cos,tan}pi(3) follow C23
- also use new .St -ieee754-2008 request in *pi.3
Reviewed by: imp
Approved by markj (mentor)
See also: D53784
Differential Revision: https://reviews.freebsd.org/D53783
(cherry picked from commit 4fcc58afbff924293c721f4c4056bb7f94580f22)