Add some comments

config.toml.example

@@ -398,6 +398,8 @@
 # Indicates whether LLD will be used to link Rust crates during bootstrap on
 # supported platforms. The LLD from the bootstrap distribution will be used
 # and not the LLD compiled during the bootstrap.
+#
+# LLD will not be used if we're cross linking or running tests.
 #use-lld = false
 
 # Indicates whether some LLVM tools, like llvm-objdump, will be made available in the

src/bootstrap/test.rs

@@ -1035,6 +1035,7 @@ impl Step for Compiletest {
         flags.push("-Zunstable-options".to_string());
         flags.push(builder.config.cmd.rustc_args().join(" "));
 
+        // Don't use LLD here since we want to test that rustc finds and uses a linker by itself.
         if let Some(linker) = builder.linker(target, false) {
             cmd.arg("--linker").arg(linker);
         }