Update -Z treat-err-as-bug description

Author: ShE3py

compiler/rustc_session/src/options.rs

@@ -1838,7 +1838,8 @@ written to standard error output)"),
     trap_unreachable: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "generate trap instructions for unreachable intrinsics (default: use target setting, usually yes)"),
     treat_err_as_bug: Option<NonZeroUsize> = (None, parse_treat_err_as_bug, [TRACKED],
-        "treat the nth error that occurs as bug (default: 0 - don't treat errors as bugs)"),
+        "treat the `val`th error that occurs as bug (default if unspecified: 0 - don't treat errors as bugs. \
+        default if specified without a value: 1 - treat the first error as bug)"),
     trim_diagnostic_paths: bool = (true, parse_bool, [UNTRACKED],
         "in diagnostics, use heuristics to shorten paths referring to items"),
     tune_cpu: Option<String> = (None, parse_opt_string, [TRACKED],