vi(1): Add URL to the vi/ex reference manual in the man page

Reported in PR 241985

The manual page references the "vi/ex reference manual" but there is no
information about where to find that document. Add a reference to the manual in
the SEE ALSO section since the Project hosts a copy of it[1].

Change sent upstream[2]

If D26158 gets reviewed and committed, we could close that PR.

[1] https://docs.freebsd.org/44doc/usd/13.viref/paper.pdf).
[2] https://github.com/lichray/nvi2/pull/85

Submitted by: freebsd@tim.thechases.com
Approved by: manpages (gbe@)
Differential Revision: https://reviews.freebsd.org/D26163