bootstrap: further clarify build --help

Author: RalfJung

src/bootstrap/flags.rs

@@ -196,13 +196,14 @@ Arguments:
         ./x.py build
         ./x.py build --stage 1
 
-    For a quick build with a usable compile, you can pass:
+    For a quick build of a usable compiler, you can pass:
 
         ./x.py build --stage 1 src/libtest
 
     This will first build everything once (like --stage 0 without further
     arguments would), and then use the compiler built in stage 0 to build
-    src/libtest and its dependencies.");
+    src/libtest and its dependencies.
+    Once this is done, build/$ARCH/stage1 contains a usable compiler.");
             }
             "test" => {
                 subcommand_help.push_str("\n