Document what distcheck is intended to exercise

src/bootstrap/src/core/build_steps/test.rs

@@ -2964,7 +2964,14 @@ impl Step for Distcheck {
         run.builder.ensure(Distcheck);
     }
 
-    /// Runs "distcheck", a 'make check' from a tarball
+    /// Runs `distcheck`, which is a collection of smoke tests:
+    ///
+    /// - Run `make check` from an unpacked dist tarball to make sure we can at the minimum run
+    ///   check steps from those sources.
+    /// - Check that selected dist components (`rust-src` only at the moment) at least have expected
+    ///   directory shape and crate manifests that cargo can generate a lockfile from.
+    ///
+    /// FIXME(#136822): dist components are under-tested.
     fn run(self, builder: &Builder<'_>) {
         builder.info("Distcheck");
         let dir = builder.tempdir().join("distcheck");

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

@@ -1,3 +1,15 @@
+# Runs `distcheck`, which is a collection of smoke tests:
+# 
+# - Run `make check` from an unpacked dist tarball to make sure we can at the
+#   minimum run check steps from those sources.
+# - Check that selected dist components at least have expected directory shape
+#   and crate manifests that cargo can generate a lockfile from.
+#
+# Refer to `src/bootstrap/src/core/build_steps/test.rs` `Distcheck::run` for
+# specifics.
+#
+# FIXME(#136822): dist components are generally under-tested.
+
 FROM ubuntu:22.04
 
 ARG DEBIAN_FRONTEND=noninteractive