pkgbase and openssl warning in releng_13 src/UPDATING

Hi all,

I noticed on 30 June the mention of a warning re: pkgbase and openssl in 
the releng_13 src/UPDATING file.

For people that don't use HEAD this may be the first time they have ever 
heard about pkgbase (which is sort-of explained at 
https://wiki.freebsd.org/PkgBase).

13-stable/releng_13 users may see this src/UPDATING message and think 
that they may need to install a package called FreeBSD-openssl to avoid 
some mysterious problem.  Then they will discover of FreeBSD-openssl 
doesn't exist because it isn't part of the ports collection.

I submit that some additional documentation (perhaps just a pointer to 
existing information) should be created somewhere to let 
13-stable/releng_13 users know a little more about this mysterious new 
"pkgbase" term.

--MCV.