Don't overwrite /usr/local/etc/pkg/repos/FreeBSD.conf

As pointed out by:
[Bug 291502](https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=291502).

I agree that we shouldn't encourage users to overwrite this file.
Since pkgbase is a tech preview and when/if it becomes the default,
overwriting this file may cause some users to have trouble. The
default FreeBSD.conf files over usr.sbin/pkg/FreeBSD* are the default
repository files, so editing them is not encouraged. FreeBSD-base is
by default set by "no" which I believe is because some people may not
want to use the pkgbase tech preview. So when the user chooses to do
so, it creates the file /usr/local/etc/pkg/repos/FreeBSD.conf by
default.

I have done some tests under a clean install over FreeBSD 15.0 with
and without package-preview. So to mitigate this problem I think until
things become more stable and or agreed it would be a great
improvement on the code and the documentation. Since right now I'm not
really familiar with how to contribute to the docs I'm writing this to
the mailing list. If someone has some spare time and this makes sense,
I think it would be great to change. If not, I'm still reading about
contributing to the documentation and getting used to sending patches
through bugzilla so, when I become familiar with it I can do it
myself.

Thanks in advance,
Candinho, G. A.