add some more summary from pr discussion

Author: Dylan-DPC

library/std/src/io/error.rs

@@ -147,7 +147,7 @@ struct Custom {
 /// In application code, use `match` for the `ErrorKind` values you are expecting; use `_` to match
 /// "all other errors".
 ///
-/// In comprehensive and thorough tests that want to verify that a test doesn't return any known incorrect error kind, 
+/// In comprehensive and thorough tests that want to verify that a test doesn't return any known incorrect error kind,
 /// you may want to cut-and-paste the current list of errors from here into your test code. This seems counterintuitive,
 /// but it will make your tests more robust. In particular, if you want to verify that your code does produce an
 /// unrecognized error kind, the robust solution is to check for all the recognized error kinds and fail in those cases.