Add docs for volume discount tier evaluation order and how ties are resolved

[Chucks1093]

Summary

When a creator's cumulative volume qualifies for multiple discount tiers, the contract applies the lowest fee rate. Contributors updating the discount tier logic need documentation explaining the evaluation order so they do not accidentally apply the wrong tier.

Scope

• Document that tiers are evaluated in ascending order of min_volume and the last matching tier wins
• Explain with a worked example: three tiers at 1000, 5000, and 10000 XLM volume — a creator at 7000 XLM qualifies for the first two and receives the tier-2 rate
• Note that updating tiers takes effect on the next trade, not retroactively
• Clarify that a creator who drops below a threshold (theoretically impossible since volume only increases) retains the earned tier

Acceptance Criteria

• Evaluation order documented with worked example
• Winning tier selection logic clearly stated
• Effect timing of tier updates documented
• Volume monotonicity property noted

ETA: 12 hours

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Coordinate on Telegram

[GBOYEE]

Hey team — I can handle this one. I'll add the docs/examples this needs. I'll open a PR referencing this shortly once assigned.

[OpensrcLord]

Hello @maintainer, I understand the expected outcome and would love to work on this. I'll provide a solution that satisfies the acceptance criteria.