Revise USS doc

[1000TurquoisePogs]

When reading the documentation I noticed we enumerate 3 of the same document links for "Introduction to z/OS Unix", and they're all for outdated versions of z/OS.

So, I figure it will age better if we just link one, the latest, and don't specify the version number, since the introduction to unix probably rarely if ever changes.

Further down, I noticed we recommend a setting for HEAP64.
That's great. But we also require HEAPPOOLS / HEAPPOOLS64 to be set certain ways. Mostly this is enforced in code ( https://github.com/zowe/launcher/blob/v3.x/staging/samplib/ZWESLSTC#L59 ) , but I just made a table out of the three to be certain.

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