f Include note on cardinality

Author: tnull

lightning/src/util/persist.rs

@@ -65,10 +65,12 @@ pub const SCORER_PERSISTENCE_KEY: &str = "scorer";
 /// Keys and namespaces are required to be valid ASCII strings in the range of
 /// [`KVSTORE_NAMESPACE_KEY_ALPHABET`] and no longer than 120 characters. Empty namespaces and
 /// sub-namespaces (`""`) are assumed to be a valid, however, if `namespace` is empty,
-/// `sub_namespace` is required to be empty, too. This means that concerns should always be separated by
-/// namespace first, before sub-namespaces are used. Note that per-namespace uniqueness needs to
-/// also hold for keys *and* namespaces/sub-namespaces in any given namespace/sub-namespace, i.e.,
-/// conflicts between keys and equally named namespaces/sub-namespaces must be avoided.
+/// `sub_namespace` is required to be empty, too. This means that concerns should always be
+/// separated by namespace first, before sub-namespaces are used. While the number of namespaces
+/// will be relatively small and is determined at compile time, there may be many sub-namespaces
+/// per namespace. Note that per-namespace uniqueness needs to also hold for keys *and*
+/// namespaces/sub-namespaces in any given namespace/sub-namespace, i.e., conflicts between keys
+/// and equally named namespaces/sub-namespaces must be avoided.
 ///
 /// **Note:** Users migrating custom persistence backends from the pre-v0.0.117 `KVStorePersister`
 /// interface can use a concatenation of `[{namespace}/[{sub_namespace}/]]{key}` to recover a `key` compatible with the