improve CI cache docs

src/tests/ci.md

@@ -322,7 +322,7 @@ Our CI workflow uses various caching mechanisms, mainly for two things:
 ### Docker images caching
 
 The Docker images we use to run most of the Linux-based builders take a *long*
-time to fully build. To speed up the build, we cache it using [Docker registry
+time to fully build. To speed up the build, we cache them using [Docker registry
 caching], with the intermediate artifacts being stored on [ghcr.io]. We also
 push the built Docker images to ghcr, so that they can be reused by other tools
 (rustup) or by developers running the Docker build locally (to speed up their
@@ -334,16 +334,32 @@ override the cache for the others. Instead, we store the images under different
 tags, identifying them with a custom hash made from the contents of all the
 Dockerfiles and related scripts.
 
+The CI calculates a hash key, so that the cache of a Docker image is
+invalidated if one of the following changes:
+
+- Dockerfile
+- Files copied into the Docker image in the Dockerfile
+- The architecture of the GitHub runner (x86 or ARM)
+
 [ghcr.io]: https://github.com/rust-lang-ci/rust/pkgs/container/rust-ci
 [Docker registry caching]: https://docs.docker.com/build/cache/backends/registry/
 
 ### LLVM caching with sccache
 
 We build some C/C++ stuff in various CI jobs, and we rely on [sccache] to cache
 the intermediate LLVM artifacts. Sccache is a distributed ccache developed by
-Mozilla, which can use an object storage bucket as the storage backend. In our
-case, the artefacts are uploaded to an S3 bucket that we control
-(`rust-lang-ci-sccache2`).
+Mozilla, which can use an object storage bucket as the storage backend.
+
+With sccache there's no need to calculate the hash key ourselves. Sccache
+invalidates the cache automatically when it detects changes to relevant inputs,
+such as the source code, the version of the compiler, and important environment
+variables.
+So we just pass the sccache wrapper on top of cargo and sccache does the rest.
+
+We store the persistent artifacts on the S3 bucket `rust-lang-ci-sccache2`. So
+when the CI runs, if sccache sees that LLVM is being compiled with the same C/C++
+compiler and the LLVM source code is the same, sccache retrieves the individual
+compiled translation units from S3.
 
 [sccache]: https://github.com/mozilla/sccache