Update -Zshow-span help message.

To clarify how it works.

Author: nnethercote

compiler/rustc_session/src/options.rs

@@ -2037,7 +2037,7 @@ written to standard error output)"),
     shell_argfiles: bool = (false, parse_bool, [UNTRACKED],
         "allow argument files to be specified with POSIX \"shell-style\" argument quoting"),
     show_span: Option<String> = (None, parse_opt_string, [TRACKED],
-        "show spans for compiler debugging (expr|pat|ty)"),
+        "show spans in the crate root file, for compiler debugging (expr|pat|ty)"),
     simulate_remapped_rust_src_base: Option<PathBuf> = (None, parse_opt_pathbuf, [TRACKED],
         "simulate the effect of remap-debuginfo = true at bootstrapping by remapping path \
         to rust's source base directory. only meant for testing purposes"),