Refine URL boundary comment in directorySize.

Clarifies that the URL leak is required by the URLEnumerator API
and has no FilePath equivalent (vs. the prior "is a Foundation API"
wording, which underspecified the constraint).

Author: chrisgeo

Sources/Services/ContainerImagesService/Server/SnapshotStore.swift

@@ -220,7 +220,7 @@ extension FileManager {
         var size: UInt64 = 0
         let resourceKeys: [URLResourceKey] = [.totalFileAllocatedSizeKey]
 
-        // URL boundary: URLResourceKey.totalFileAllocatedSizeKey is a Foundation API
+        // URL boundary: required by URLEnumerator API; no FilePath equivalent
         let dirURL = URL(fileURLWithPath: dir.string, isDirectory: true)
         guard
             let enumerator = self.enumerator(