Add comment about disabling LLVM by setting codegen-backends empty

Author: bjorn3

config.toml.example

@@ -330,8 +330,8 @@
 # This is an array of the codegen backends that will be compiled for the rustc
 # that's being compiled. The default is to only build the LLVM codegen backend,
 # but you can also optionally enable the "emscripten" backend for asm.js or
-# make this an empty array (but that probably won't get too far in the
-# bootstrap)
+# make this an empty array (that will disable LLVM, but bootstrap will fail at
+# the time the final rustc binary is built for stage 1)
 #codegen-backends = ["llvm"]
 
 # This is the name of the directory in which codegen backends will get installed