Add note about caching to ci efficiency doc

doc/ci/pipelines/pipeline_efficiency.md

@@ -189,8 +189,12 @@ be more efficient, but can also make pipelines harder to understand and analyze.
 
 ### Caching
 
-Another optimization method is to use [caching](../caching/index.md) between jobs and stages,
-for example [`/node_modules` for NodeJS](../caching/index.md#caching-nodejs-dependencies).
+Another optimization method is to [cache](../caching/index.md) dependencies. If your
+dependencies change rarely, like [NodeJS `/node_modules`](../caching/index.md#caching-nodejs-dependencies),
+caching can make pipeline execution much faster.
+
+You can use [`cache:when`](../yaml/README.md#cachewhen) to cache downloaded dependencies
+even when a job fails.
 
 ### Docker Images