Default enable_memory_uss_pss check to true + add docs

What does this MR do?

To roll out the new Ruby USS+PSS memory metrics, in #216597 (closed) we had used an env var instead of a feature toggle.

Now that we've tested the change to be OK on a few selected nodes, we feel comfortable to roll it out by default to all nodes with the next deploy, but maintaining the option to turn the metric off again with the same env var.

Also, since it will be live for everyone, this adds the necessary documentation.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

See #216597 (closed) and related issues for how this was rolled out and verified.

Refs #216597 (closed)