Merge branch 'clarify_ci_caching' into 'master'

Clarify cache behavior

After chatting with @ayufan about CI caching, I added some info to the documentation. This should make it clear how caching currently works.

cc/ @ayufan Is this accurate?

See merge request !2072

doc/ci/yaml/README.md

@@ -113,6 +113,9 @@ The YAML-defined variables are also set to all created service containers, thus
 
 ### cache
 `cache` is used to specify list of files and directories which should be cached between builds.
+Caches are stored according to the branch/ref and the job name. Caches are not
+currently shared between different job names or between branches/refs. This means
+caching will benefit you if you push subsequent commits to an existing feature branch.
 
 **The global setting allows to specify default cached files for all jobs.**