improve --stage documentation

Author: RalfJung

src/bootstrap/flags.rs

@@ -121,7 +121,10 @@ To learn more about a subcommand, run `./x.py <subcommand> -h`"
         opts.optmulti("", "exclude", "build paths to exclude", "PATH");
         opts.optopt("", "on-fail", "command to run on failure", "CMD");
         opts.optflag("", "dry-run", "dry run; don't build anything");
-        opts.optopt("", "stage", "stage to build", "N");
+        opts.optopt("", "stage",
+            "stage to build (indicates compiler to use/test, e.g. stage 0 uses the \
+             bootstrap compiler, stage 1 the stage 0 rustc artifacts, etc.)",
+            "N");
         opts.optmulti("", "keep-stage", "stage(s) to keep without recompiling", "N");
         opts.optopt("", "src", "path to the root of the rust checkout", "DIR");
         opts.optopt("j", "jobs", "number of jobs to run in parallel", "JOBS");