payout: qualify the single-writer claim for pool_status.json

The comment justifying the absence of a version guard said RefreshStats
"runs only on the event loop". That was true when it was written and is
no longer: the payout-ledger self-test drives RefreshStats directly, to
check that no payout mutex is held across the write.

The conclusion still holds -- the test calls it from one thread at a
time with the pool not running, so pool_status.json still has a single
writer -- but the reason a reader would check is now stated, along with
what would invalidate it. A claim in a comment that has quietly stopped
being true is how 33f2e8a came to assert that no thread waits on I/O
holding gPayoutLedgerMutex on the commit that made it do so.

Property: comment only.

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