f Qualify payment cache warmup

Describe the built-in backend assumption and limit the cache-hit
claim to startup, before normal activity can evict warmed entries.

Co-Authored-By: HAL 9000

Author: tnull

src/config.rs

@@ -59,9 +59,10 @@ pub(crate) const PAYMENT_CACHE_CAPACITY: NonZeroUsize = NonZeroUsize::new(1000).
 
 // The number of payments we read into the cache when starting up.
 //
-// This matches the storage backends' page size, so warming the cache costs a single page listing
-// and one batch of reads, and the first page of `Node::list_payments` is served without going to
-// the store at all. The remaining capacity fills as payments are used.
+// This matches the built-in storage backends' page size, so warming the cache costs a single page
+// listing and one batch of reads. Immediately after startup, a first-page `Node::list_payments`
+// call reads only its keys from storage; the payment bodies come from the cache. Later activity
+// may displace those entries.
 pub(crate) const PAYMENT_CACHE_WARMUP_COUNT: NonZeroUsize = NonZeroUsize::new(50).unwrap();
 
 // The default {Esplora,Electrum} client timeout we're using.