[Cache] update docs about encryption using SodiumMarshaller

Author: pableu

cache.rst

@@ -718,9 +718,19 @@ Clear all caches everywhere:
 Encrypting the Cache
 --------------------
 
+.. versionadded:: 5.1
+
+    :class:`Symfony\\Component\\Cache\\Marshaller\\SodiumMarshaller` has been
+    introduced in Symfony 5.1.
+
 To encrypt the cache using ``libsodium``, you can use the
 :class:`Symfony\\Component\\Cache\\Marshaller\\SodiumMarshaller`.
 
+.. note::
+
+    This will encrypt the values of the cache items, but not the cache keys. Be
+    careful not the leak sensitive data in the keys.
+
 Generate a key:
 
 .. code-block:: terminal
@@ -828,5 +838,5 @@ for reading and writing, and the additional key(s) will only be used for reading
             ->addArgument(['env(base64:CACHE_DECRYPTION_KEY)', 'env(base64:OLD_CACHE_DECRYPTION_KEY)'])
             ->addArgument(service('@Symfony\Component\Cache\Marshaller\SodiumMarshaller.inner'));
 
-Once all cache entries encrypted with the old key have expired, you can remove
+Once all cache items encrypted with the old key have expired, you can remove
 `OLD_CACHE_DECRYPTION_KEY` completely.