Be explicit about impact of not writing caches

A follow-up to https://github.com/containers/storage/pull/2031 .

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

pkg/chunked/cache_linux.go

@@ -289,8 +289,10 @@ func (c *layersCache) load() error {
 		}
 
 		if r.ReadOnly {
-			// if the layer is coming from a read-only store, do not attempt
+			// If the layer is coming from a read-only store, do not attempt
 			// to write to it.
+			// Therefore,we won’t find any matches in read-only-store layers,
+			// unless the read-only store layer comes prepopulated with cacheKey data.
 			continue
 		}