Docs(crates/cargo-test-*): Add external usage warning

As discussed in #10147.

Signed-off-by: Paul Mabileau <paulmabileau@hotmail.fr>

crates/cargo-test-macro/README.md

@@ -0,0 +1,5 @@
+WARNING: You might not want to use this outside of Cargo.
+
+* This is designed for testing Cargo itself. Use at your own risk.
+* No guarantee on any stability across versions.
+* No feature request would be accepted unless proved useful for testing Cargo.

crates/cargo-test-macro/src/lib.rs

@@ -1,3 +1,14 @@
+//! # Cargo test macro.
+//!
+//! This is meant to be consumed alongside `cargo-test-support`. See
+//! <https://rust-lang.github.io/cargo/contrib/> for a guide on writing tests.
+//!
+//! WARNING: You might not want to use this outside of Cargo.
+//!
+//! * This is designed for testing Cargo itself. Use at your own risk.
+//! * No guarantee on any stability across versions.
+//! * No feature request would be accepted unless proved useful for testing Cargo.
+
 use proc_macro::*;
 use std::path::Path;
 use std::process::Command;

crates/cargo-test-support/README.md

@@ -0,0 +1,5 @@
+WARNING: You might not want to use this outside of Cargo.
+
+* This is designed for testing Cargo itself. Use at your own risk.
+* No guarantee on any stability across versions.
+* No feature request would be accepted unless proved useful for testing Cargo.

crates/cargo-test-support/src/lib.rs

@@ -1,6 +1,12 @@
 //! # Cargo test support.
 //!
 //! See <https://rust-lang.github.io/cargo/contrib/> for a guide on writing tests.
+//!
+//! WARNING: You might not want to use this outside of Cargo.
+//!
+//! * This is designed for testing Cargo itself. Use at your own risk.
+//! * No guarantee on any stability across versions.
+//! * No feature request would be accepted unless proved useful for testing Cargo.
 
 #![allow(clippy::disallowed_methods)]
 #![allow(clippy::print_stderr)]