Improve reasoning and examples for F.48 (#2100)

* fix incorrect reason for F.48

* distinguish rvo and nrvo

* the issue is about local, so limiting example to local

---------

Co-authored-by: Sergey Zubkov <[email protected]>

CppCoreGuidelines.md

@@ -3932,21 +3932,24 @@ value) of any assignment operator.
 
 ##### Reason
 
-With guaranteed copy elision, it is now almost always a pessimization to expressly use `std::move` in a return statement.
+Returning a local variable implicitly moves it anyway.
+An explicit `std::move` is always a pessimization, because it prevents Return Value Optimization (RVO),
+which can eliminate the move completely. 
 
 ##### Example, bad
 
-    S f()
+    S bad()
     {
       S result;
       return std::move(result);
     }
 
 ##### Example, good
 
-    S f()
+    S good()
     {
       S result;
+      // Named RVO: move elision at best, move construction at worst
       return result;
     }