Update treat-err-as-bug help text

Author: estebank

src/librustc/session/config.rs

@@ -1244,7 +1244,7 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
     no_codegen: bool = (false, parse_bool, [TRACKED],
         "run all passes except codegen; no output"),
     treat_err_as_bug: Option<usize> = (None, parse_treat_err_as_bug, [TRACKED],
-        "treat all errors that occur as bugs"),
+        "treat error number `val` that occurs as bug"),
     report_delayed_bugs: bool = (false, parse_bool, [TRACKED],
         "immediately print bugs registered with `delay_span_bug`"),
     external_macro_backtrace: bool = (false, parse_bool, [UNTRACKED],