Add an initial amount of documentation of registry based

Add an initial amount of documentation of registry based
configuration to smb.conf(5). This explains registry shares
and registry global options and mentions "net conf".

This will be in 3.0.26, but Jerry told me to commit this
anyways. The plan is to branch the doc for 3.0.25b from r1126 (say).

I could not check the result since I could not get the
manpages built correctly on any system I tried. So the
output needs to be checked - sorry, I will fix my
environment later...

Michael