erf.3: Add a STANDARDS section

Add a STANDARDS section for the erf(3) manual page.

PR: 273413
Reviewed by: pauamma_gundo.com
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D44140

(cherry picked from commit 16c3d7e1fa150ee136b910448cf431e8deab5293)