docs(fee-abstraction): clarify expiration_ledger in collect_fee's Lazy branch

[Eras256]

Summary

collect_fee and collect_fee_and_invoke document expiration_ledger
with identical prose ("the ledger sequence at which the approval
expires"). That's accurate for collect_fee_and_invoke, where the value
is threaded into user_args_for_auth as a freshness bound on the signed
call itself, but it reads as "the allowance's expiration" when
collect_fee is read (or called) in isolation, since it's the only
place that phrasing appears for a function with no require_auth of
its own.

That ambiguity produced a real filed-and-withdrawn finding, #840 (fix
attempt at #844): a reproduction called collect_fee directly with a
genuinely non-expired 100-unit allowance and still hit
InvalidExpirationLedger, read at the time as a bug. @brozorec's
closing explanation on #840 was correct and is not being reopened here:
in the Lazy branch, once an existing allowance already covers
max_fee_amount, expiration_ledger is validated via
validate_expiration_ledger as a freshness bound on the call, wholly
independent of the token's own allowance expiry. This PR is the doc
clarification promised in that thread's closing comment, so the next
reader doesn't hit the same misreading.

Changes

Three lines added to collect_fee's own # Arguments doc comment,
nothing else touched.

Test plan

• Doc-only change, no behavior touched.
• cargo doc -p stellar-fee-abstraction --no-deps builds clean locally.

Disclosure: drafted with AI assistance under my direction and reviewed
by hand.

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ℹ️ Recent review info

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Reviewing files that changed from the base of the PR and between f11f8c0 and 6966c68.

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• packages/fee-abstraction/src/storage.rs

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Walkthrough

The collect_fee documentation now clarifies how expiration_ledger behaves in the Lazy approval branch. No functional code changes were made.

Changes

Fee collection documentation

Layer / File(s)	Summary
Expiration ledger clarification packages/fee-abstraction/src/storage.rs	The expiration_ledger documentation now states that it provides a freshness bound on the call when an existing allowance covers max_fee_amount. It is not read from the token’s on-chain allowance expiry.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~3 minutes

Change: Other

Suggested reviewers: brozorec

Merge Risk: ⚪ Minimal · up to 6966c

This documentation clarification does not alter fee collection behavior and is safe to merge.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 1…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely identifies the documentation change to expiration_ledger in collect_fee’s Lazy branch.
Description check	✅ Passed	The description is detailed and relevant. It explains the ambiguity, scope, implementation change, and test plan. The issue placeholder remains unfilled, and the template checklist is omitted, but the…

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