docs: mention build cache in docker system prune description

The synopsis only listed containers, networks, images, and volumes,
but the command always prunes unused build cache (including BuildKit
cache mounts). Align the description with the interactive warning.

Signed-off-by: Dean Chen <862469039@qq.com>

Author: locker95

docs/reference/commandline/system_prune.md

@@ -18,7 +18,13 @@ Remove unused data
 ## Description
 
 Remove all unused containers, networks, images (both dangling and unused),
-and optionally, volumes.
+unused [build cache](https://docs.docker.com/build/cache/), and optionally,
+volumes.
+
+Build cache is always eligible for pruning with this command (including BuildKit
+cache mounts created with `RUN --mount=type=cache`). Use
+[`docker builder prune`](builder_prune.md) if you only want to reclaim build
+cache without removing containers, networks, or images.
 
 ## Examples