Add vi/ex reference manual URL to man page

[fernape]

Reported in FreeBSD's Bugzilla 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 FreeBSD Project hosts a copy of it.

Test:

• mandoc -T lint ./vi.1 && igor ./vi.1 seem happy
• man ./vi.1 renders the page properly

[lichray]

Would you like to copy the .Nm vi Ns / Ns Nm ex formatting?

[fernape]

Would you like to copy the .Nm vi Ns / Ns Nm ex formatting?

That would mean dropping .%T and .Rs block, right? Otherwise I get multiple WARNINGS:

mandoc: ./vi.1:2747:2: WARNING: skipping empty macro: %T
mandoc: ./vi.1:2748:8: WARNING: invalid content in Rs block: Ns
mandoc: ./vi.1:2748:11: WARNING: invalid content in Rs block: text
mandoc: ./vi.1:2748:13: WARNING: invalid content in Rs block: Ns
mandoc: ./vi.1:2748:16: WARNING: invalid content in Rs block: Nm

Thanks for looking at this.

[lichray]

Nevermind, it's good.