wg.4: Add some enhancements

• add a SPDX-License-Identifier
• rename the title of the man page
• use better grammar in some places
• reword 'IPs' to 'IP addresses'
• add a missing word in the AUTHORS section
• use '.An -nosplit' in the AUTHORS section
• Xr ipsec and ovpn

Reviewed by: pauamma, mhorne
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D37205