Rollup merge of #85988 - jyn514:ninja-error, r=joshtriplett

Note that `ninja = false` goes under `[llvm]`

Addresses https://github.com/rust-lang/rust/issues/84938#issuecomment-852448332 - `@kornelski` does this look good?

r? `@Mark-Simulacrum` cc `@joshtriplett`

src/bootstrap/lib.rs

@@ -1366,7 +1366,7 @@ impl Build {
                 eprintln!(
                     "
 Couldn't find required command: ninja
-You should install ninja, or set ninja=false in config.toml
+You should install ninja, or set `ninja=false` in config.toml in the `[llvm]` section.
 "
                 );
                 std::process::exit(1);