Improve documentation for tools profile

Make the build process more beginner friendly:

 - Include information explaining that the stage2 toolchain should be
   used (and not the stage1 toolchain) due to the `download-rustc`
   setting.

 - Display a message when the user runs `x setup tools` explaining that
   they should use the stage2 toolchain.

src/bootstrap/defaults/config.tools.toml

@@ -9,6 +9,8 @@ debug-logging = true
 incremental = true
 # Download rustc from CI instead of building it from source.
 # This cuts compile times by almost 60x, but means you can't modify the compiler.
+# Using these defaults will download the stage2 compiler (see `download-rustc`
+# setting) and the stage2 toolchain should therefore be used for these defaults.
 download-rustc = "if-unchanged"
 
 [build]

src/bootstrap/setup.rs

@@ -176,6 +176,14 @@ pub fn setup(config: &Config, profile: Profile) {
         );
     }
 
+    if profile == Profile::Tools {
+        eprintln!();
+        eprintln!(
+            "note: the `tools` profile sets up the `stage2` toolchain (use \
+            `rustup toolchain link 'name' host/build/stage2` to use rustc)"
+        )
+    }
+
     let path = &config.config.clone().unwrap_or(PathBuf::from("config.toml"));
     setup_config_toml(path, profile, config);
 }