diff -u b/core/core.api.php b/core/core.api.php
--- b/core/core.api.php
+++ b/core/core.api.php
@@ -533,10 +533,15 @@
  * variation of the contextual data was used in the calculatation. The next time
  * the computed data is needed, if the context matches that for an existing
  * cached data set, the cached data can be reused; if no context matches, a new
- * data set can be calculated and cached for later use. See
+ * data set can be calculated and cached for later use.
+ *
+ * Cache contexts are services tagged with 'cache.context', whose classes
+ * implement \Drupal\Core\Cache\Context\CacheContextInterface. See
  * https://www.drupal.org/developing/api/8/cache/contexts for more information
  * on cache contexts, including a list of the contexts that exist in Drupal
- * core, and information on how to define your own contexts.
+ * core, and information on how to define your own contexts. See the
+ * @link container Services and the Dependency Injection Container @endlink
+ * topic for more information about services.
  *
  * Typically, the cache context is specified as part of the #cache property
  * of a render array; see the Caching section of the