Document upstream tooling wrappers (bazelbuild#2890)

docs/src/SUMMARY.md

@@ -18,3 +18,4 @@
 - [External Crates](./external_crates.md)
     - [crate_universe (bzlmod)](crate_universe_bzlmod.md)
     - [crate_universe (WORKSPACE)](crate_universe.md)
+- [Upstream Tooling](./upstream_tooling.md)

docs/src/upstream_tooling.md

@@ -0,0 +1,13 @@
+# Upstream Tooling
+
+rules_rust manages versions of things like rustc. If you want to manually run upstream tooling configured at the versions, plugins and such that rules_rust has configured, rules_rust exposes these as targets in `@rules_rust//tools/upstream_wrapper`:
+
+```console
+% bazel query @rules_rust//tools/upstream_wrapper
+@rules_rust//tools/upstream_wrapper:cargo
+@rules_rust//tools/upstream_wrapper:cargo_clippy
+@rules_rust//tools/upstream_wrapper:rustc
+@rules_rust//tools/upstream_wrapper:rustfmt
+```
+
+You can run them via `bazel run`, e.g. `bazel run @rules_rust//tools/upstream_wrapper:cargo -- check`.