Added example for SF.21 (isocpp#1890)

* Example for SF.21

* Update isocpp.dic

* remove space

* revert isocpp.dic change

Author: bgloyer

CppCoreGuidelines.md

@@ -19558,7 +19558,26 @@ It is almost always a bug to mention an unnamed namespace in a header file.
 
 ##### Example
 
-    ???
+    // file foo.h:
+    namespace
+    {
+        const double x = 1.234;  // bad
+
+        double foo(double y)     // bad
+        {
+            return y + x;
+        }
+    }
+
+    namespace Foo
+    {
+        inline constexpr double x = 1.234; // good
+
+        inline double foo(double y)        // good
+        {
+            return y + x;
+        }
+    }
 
 ##### Enforcement