Clarify docs

Author: tmandry

src/ci/docker/host-x86_64/x86_64-gnu-integration/Dockerfile

@@ -1,5 +1,6 @@
 # This job builds a toolchain capable of building Fuchsia, and then builds
-# Fuchsia. See the build-fuchsia.sh script in this directory for more details.
+# Fuchsia as an integration test of the toolchain. See the build-fuchsia.sh
+# script in this directory for more details.
 
 FROM ubuntu:22.04