describe -Zmir-include-spans more accurately

Author: lqd

compiler/rustc_session/src/options.rs

@@ -1833,7 +1833,8 @@ options! {
         enable unsound (known-buggy and hence usually disabled) passes without further warning! \
         Passes that are not specified are enabled or disabled by other flags as usual."),
     mir_include_spans: Option<bool> = (None, parse_opt_bool, [UNTRACKED],
-        "use line numbers relative to the function in mir pretty printing"),
+        "include extra comments in mir pretty printing, like line numbers and statement indices, \
+         details about types, etc. (default: no)"),
     mir_keep_place_mention: bool = (false, parse_bool, [TRACKED],
         "keep place mention MIR statements, interpreted e.g., by miri; implies -Zmir-opt-level=0 \
         (default: no)"),