core/core.api.php                                 | 3 +++
 core/lib/Drupal/Core/Cache/BackendChain.php       | 2 ++
 core/lib/Drupal/Core/Cache/ChainedFastBackend.php | 9 +++++++++
 3 files changed, 14 insertions(+)

diff --git a/core/core.api.php b/core/core.api.php
index de34440..baf7940 100644
--- a/core/core.api.php
+++ b/core/core.api.php
@@ -568,6 +568,9 @@
  *  $settings['cache']['default'] = 'cache.custom';
  * @endcode
  *
+ * Finally, you can chain multiple cache backends together, see
+ * \Drupal\Core\Cache\ChainedFastBackend and \Drupal\Core\Cache\BackendChain.
+ *
  * @see https://www.drupal.org/node/1884796
  * @}
  */
diff --git a/core/lib/Drupal/Core/Cache/BackendChain.php b/core/lib/Drupal/Core/Cache/BackendChain.php
index c861b74..2e17d67 100644
--- a/core/lib/Drupal/Core/Cache/BackendChain.php
+++ b/core/lib/Drupal/Core/Cache/BackendChain.php
@@ -20,6 +20,8 @@
  * to ensure fast retrieval on the next request. On cache sets and deletes, both
  * backends will be invoked to ensure consistency.
  *
+ * @see \Drupal\Core\Cache\ChainedFastBackend
+ *
  * @ingroup cache
  */
 
diff --git a/core/lib/Drupal/Core/Cache/ChainedFastBackend.php b/core/lib/Drupal/Core/Cache/ChainedFastBackend.php
index ce5ddd2..43cda30 100644
--- a/core/lib/Drupal/Core/Cache/ChainedFastBackend.php
+++ b/core/lib/Drupal/Core/Cache/ChainedFastBackend.php
@@ -39,6 +39,15 @@
  * Because this backend will mark all the cache entries in a bin as out-dated
  * for each write to a bin, it is best suited to bins with fewer changes.
  *
+ * Note that this is designed specifically for combining a fast inconsistent
+ * cache back-end with a slower consistent cache back-end. To still function
+ * correctly, it needs to do a consistency check (see the "last write timestamp"
+ * logic). This contrasts with \Drupal\Core\Cache\BackendChain, which assumes
+ * both chained cache back-ends are consistent, thus a consistency check being
+ * pointless.
+ *
+ * @see \Drupal\Core\Cache\BackendChain
+ *
  * @ingroup cache
  */
 class ChainedFastBackend implements CacheBackendInterface, CacheTagsInvalidatorInterface {