Update LLVM comments in config.codegen.toml

This comment became obsolete after rust-lang#116881 switched over to
`download-ci-llvm = "if-unchanged"`.

src/bootstrap/defaults/config.codegen.toml

@@ -16,9 +16,9 @@ backtrace-on-ice = true
 lto = "off"
 
 [llvm]
-# build llvm from source
+# Will download LLVM from CI if available on your platform.
 download-ci-llvm = "if-unchanged"
-# This enables debug-assertions in LLVM,
+# This enables debug-assertions in LLVM (or downloads a CI build with assertions),
 # catching logic errors in codegen much earlier in the process.
 assertions = true
 # enable warnings during the llvm compilation