Clarify intermediate struct docs

commons/src/layer/cache_buddy.rs

@@ -174,9 +174,9 @@ where
     }
 }
 
-/// Either contains metadata or a message describing the state
+/// Either contains (old) metadata or a message describing the state
 ///
-/// Why: The `CachedLayerDefinition` allows returning information about the cache state
+/// Why: The [`CachedLayerDefinition`] allows returning information about the cache state
 /// from either `invalid_metadata_action` or `restored_layer_action` functions.
 ///
 /// Because the function returns only a single type, that type must be the same for
@@ -189,11 +189,10 @@ where
 /// to inspect the previous metadata they can match on the enum and extract
 /// what they need.
 ///
+/// When produced using functions in this module:
+///
 /// - Will only ever contain metadata when the cache is retained.
 /// - Will contain a message when the cache is cleared, describing why it was cleared.
-///   It is also allowable to return a message when the cache is retained, and the
-///   message describes the state of the cache. (i.e. because a message is returned
-///   does not guarantee the cache was cleared).
 pub enum Meta<M> {
     Message(String),
     Data(M),