symlink.2: document EOPNOTSUPP

When the file system does not support symbolic links (like in the case
of MSDOS), symlink() returns -1 and sets errno to EOPNOTSUPP.
Document this behavior.

Reviewed by: glebius, markj
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D49803