[gn build] add a comment to the goma_dir arg

Author: nico

llvm/utils/gn/build/toolchain/BUILD.gn

@@ -2,6 +2,7 @@ import("//llvm/utils/gn/build/toolchain/compiler.gni")
 
 declare_args() {
   # If is_goma is true, the location of the goma client install.
+  # Set this to the output of `goma_ctl goma_dir`.
   if (host_os == "win") {
     goma_dir = "C:/src/goma/goma-win64"
   } else {