#737 Change instructions for installing STAF on OS/400

[Sharon Lucas]

The instructions for installing STAF on OS/400 do not
say to install STAF using STAFInst. Instead, it just
runs STAF from the untarred files.

The instructions should be changed to run STAFInst
(just like on other platforms). That way, you can
select options like IPv4/IPv6 support and you'll get
the STAFEnv.sh script to set up your environment
variables to run STAF.

I have verified that STAFInst runs fine on OS/400. In
fact, the STAF Upgrade library uses STAFInst when
upgrading the version of STAF installed on a OS/400
machine.

[Sharon Lucas]

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Updated instructions in docs/userguide/Install.script.