two servers on the same port

I just spent a couple of days trying to figure out why I couldn't start
two servers on the same port, even though I was configuring separate
listen_address values. I kept gettting errors about shmget failing with
"could not create shared memory segment: Invalid argument".

I finally noticed that the shared memory key mentioned in the error when
starting the second server was the same as what the first server was
using, which appeared to be generated based off of the port number.

Sure enough when I changed the port, it used a different shared memory
key and started right up. After searching around on the web a bit
I found some pages that suggested running under different userids
might be necessary. So, I tried that, and changed the port back to the
standard 5432, and it started up.

Anyway, everything seems to be working fine, but I figured this info
should be a bit easier to find, so here's a couple patches to the
documentation to mention how this works.

eric