Tighten up CP.1 (#1405)

* Tighten up CP.1

* balanced verb usage in first sentence
* changed third sentence to "libraries not using threads", as I
  believe this was the original author's intended meaning.
* clarified "this" in fourth sentence
* cut wordiness of "thanks to the magic of cut-and-paste", as it
  added no value
* changed "Example" heading to "Example, bad"
* added "bad:" comment above statics in the example
* added an explanatory sentence immediately after the example
* changed "works perfectly in a single-threaded" after example to
  "works as intended in a single threaded".  Also balanced the
  structure of the two comma separated phrases inside this sentence.
* strengthened parenthetical explanation in second bullet of "could
  be made safe" section

* Correct grammar mistake pointed out by @cubbimew

* Remove specific cache details in CP.1 per @hsutter's request

CppCoreGuidelines.md

@@ -13797,16 +13797,17 @@ Concurrency and parallelism rule summary:
 
 ##### Reason
 
-It is hard to be certain that concurrency isn't used now or will be sometime in the future.
+It's hard to be certain that concurrency isn't used now or won't be used sometime in the future.
 Code gets reused.
-Libraries using threads may be used from some other part of the program.
-Note that this applies most urgently to library code and least urgently to stand-alone applications.
-However, thanks to the magic of cut-and-paste, code fragments can turn up in unexpected places.
+Libraries not using threads may be used from some other part of a program that does use threads.
+Note that this rule applies most urgently to library code and least urgently to stand-alone applications.
+However, over time, code fragments can turn up in unexpected places.
 
-##### Example
+##### Example, bad
 
     double cached_computation(double x)
     {
+        // bad: these two statics cause data races in multi-threaded usage
         static double cached_x = 0.0;
         static double cached_result = COMPUTATION_OF_ZERO;
         double result;
@@ -13819,8 +13820,6 @@ However, thanks to the magic of cut-and-paste, code fragments can turn up in une
         return result;
     }
 
-Although `cached_computation` works perfectly in a single-threaded environment, in a multi-threaded environment the two `static` variables result in data races and thus undefined behavior.
-
 There are several ways that this example could be made safe for a multi-threaded environment:
 
 * Delegate concurrency concerns upwards to the caller.