pthread_setschedparam(3): document EPERM return

In kern_sched_setparam(), before setting any parameters, p_cansched() is
called to check that the thread has appropriate privileges.

PR: 175687
Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D37020

(cherry picked from commit 04620006b9116049e9e2f27d74417d2d84dc8d10)