docs/55526: Update the branches pic and ascii art for the releng article

[Gary W. Swearingen]

bmah at FreeBSD.org (Bruce A. Mah) writes:

> Good point.  This is a problem especially near the right side of the 
> figure, where we have parallel HEAD and RELENG branches.  I'll see if I 
> can connect these up in a non-confusing, non-ugly way.

It looks real good the way it is.  Maybe put tails on the top of
the long vertical lines to match the corresponding heads.

Some people might scratch their head about why many releases are in a
long line of boxes (eg, 4.0 - 4.2), while others are in a short
offshoot.  The "key" box could be made taller to explain it.  ("Recent
releases are on short 'release' branches which are limited mostly to
security patches."  Something like that.)