Document scheduler memory fencing flags

[shawnwang18]

What changed

• Document the experimental scheduler memory-fencing threshold flag.
• Explain the threshold values and the optional slack-windows control.

Why

OpenXLA commit openxla/xla@26d6634 introduced scheduler memory fencing to limit peak memory when the latency-hiding scheduler keeps large buffers live by deferring their users.

Impact

JAX/XLA users can discover how to enable and tune the feature when latency-hiding scheduling causes excessive peak GPU memory use or OOMs.

Validation

• Reviewed the rendered Markdown structure.
• Ran git diff --check.

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