Remove a broken example, and copyedit. (#1637)

Using `this` in a free function makes the program ill-formed;
it doesn't magically give you a `std::thread*`.

Author: Quuxplusone

CppCoreGuidelines.md

@@ -14516,7 +14516,7 @@ Flag attempts to pass local variables to a thread that might `detach()`.
 
 A `joining_thread` is a thread that joins at the end of its scope.
 Detached threads are hard to monitor.
-It is harder to ensure absence of errors in detached threads (and potentially detached threads)
+It is harder to ensure absence of errors in detached threads (and potentially detached threads).
 
 ##### Example, bad
 
@@ -14549,45 +14549,22 @@ It is harder to ensure absence of errors in detached threads (and potentially de
         t2.join();
     }  // one bad bug left
 
-
-##### Example, bad
-
-The code determining whether to `join()` or `detach()` may be complicated and even decided in the thread of functions called from it or functions called by the function that creates a thread:
-
-    void tricky(thread* t, int n)
-    {
-        // ...
-        if (is_odd(n))
-            t->detach();
-        // ...
-    }
-
-    void use(int n)
-    {
-        thread t { tricky, this, n };
-        // ...
-        // ... should I join here? ...
-    }
-
-This seriously complicates lifetime analysis, and in not too unlikely cases makes lifetime analysis impossible.
-This implies that we cannot safely refer to local objects in `use()` from the thread or refer to local objects in the thread from `use()`.
-
 ##### Note
 
 Make "immortal threads" globals, put them in an enclosing scope, or put them on the free store rather than `detach()`.
-[don't `detach`](#Rconc-detached_thread).
+[Don't `detach`](#Rconc-detached_thread).
 
 ##### Note
 
-Because of old code and third party libraries using `std::thread` this rule can be hard to introduce.
+Because of old code and third party libraries using `std::thread`, this rule can be hard to introduce.
 
 ##### Enforcement
 
 Flag uses of `std::thread`:
 
-* Suggest use of `gsl::joining_thread`.
+* Suggest use of `gsl::joining_thread` or C++20 `std::jthread`.
 * Suggest ["exporting ownership"](#Rconc-detached_thread) to an enclosing scope if it detaches.
-* Seriously warn if it is not obvious whether if joins of detaches.
+* Warn if it is not obvious whether a thread joins or detaches.
 
 ### <a name="Rconc-detached_thread"></a>CP.26: Don't `detach()` a thread