core/state/snapshot: clarify comment about snapshot repair (#23305)

Co-authored-by: Tyler Chambers <me@tylerchambers.net>
Co-authored-by: Felix Lange <fjl@twurst.com>

core/state/snapshot/snapshot.go

@@ -169,11 +169,17 @@ type Tree struct {
 // store (with a number of memory layers from a journal), ensuring that the head
 // of the snapshot matches the expected one.
 //
-// If the snapshot is missing or the disk layer is broken, the entire is deleted
-// and will be reconstructed from scratch based on the tries in the key-value
-// store, on a background thread. If the memory layers from the journal is not
-// continuous with disk layer or the journal is missing, all diffs will be discarded
-// iff it's in "recovery" mode, otherwise rebuild is mandatory.
+// If the snapshot is missing or the disk layer is broken, the snapshot will be
+// reconstructed using both the existing data and the state trie.
+// The repair happens on a background thread.
+// 
+// If the memory layers in the journal do not match the disk layer (e.g. there is
+// a gap) or the journal is missing, there are two repair cases:
+// 
+// - if the 'recovery' parameter is true, all memory diff-layers will be discarded.
+//   This case happens when the snapshot is 'ahead' of the state trie.
+// - otherwise, the entire snapshot is considered invalid and will be recreated on
+//   a background thread.
 func New(diskdb ethdb.KeyValueStore, triedb *trie.Database, cache int, root common.Hash, async bool, rebuild bool, recovery bool) (*Tree, error) {
 	// Create a new, empty snapshot tree
 	snap := &Tree{