improving SF.6

In respose to  #725

CppCoreGuidelines.md

@@ -16399,7 +16399,7 @@ Source file rule summary:
 * [SF.3: Use `.h` files for all declarations used in multiple source files](#Rs-declaration-header)
 * [SF.4: Include `.h` files before other declarations in a file](#Rs-include-order)
 * [SF.5: A `.cpp` file must include the `.h` file(s) that defines its interface](#Rs-consistency)
-* [SF.6: Use `using namespace` directives for transition, for foundation libraries (such as `std`), or within a local scope](#Rs-using)
+* [SF.6: Use `using namespace` directives for transition, for foundation libraries (such as `std`), or within a local scope (only)](#Rs-using)
 * [SF.7: Don't write `using namespace` in a header file](#Rs-using-directive)
 * [SF.8: Use `#include` guards for all `.h` files](#Rs-guards)
 * [SF.9: Avoid cyclic dependencies among source files](#Rs-cycles)
@@ -16602,19 +16602,60 @@ The argument-type error for `bar` cannot be caught until link time because of th
 
 ???
 
-### <a name="Rs-using"></a>SF.6: Use `using namespace` directives for transition, for foundation libraries (such as `std`), or within a local scope
+### <a name="Rs-using"></a>SF.6: Use `using namespace` directives for transition, for foundation libraries (such as `std`), or within a local scope (only)
 
 ##### Reason
 
- ???
+ `using namespace` can lead to name clashes, so it should be used sparingly.
+ However, it is not always possible to qualify every name from a namespace in user code (e.g., during transition)
+ and sometimes a namespace is so fundamental and prevalent in a code base, that consistent qualification would be verbose and distracting.
 
 ##### Example
 
-    ???
+    #include<string>
+    #include<vector>
+    #include<iostream>
+    #include<memory>
+    #include<algorithm>
+
+    using namespace std;
+
+    // ...
+
+Here (obviously), the standard library is used pervasively and apparantly no other library is used, so requiring `std::` everywhere
+could be distracting.
+
+##### Example
+
+The use of `using namespace std;` leaves the programmer open to a name clash with a name from the standard library
+
+    #include<cmath>
+    using namespace std;
+
+    int g(int x)
+    {
+        int sqrt = 7;
+        // ...
+        return sqrt(x); // error
+    }
+
+However, this is not particularly likely to lead to a resolution that is not an error and
+people who use `using namespace std` are supposed to know about `std` and about this risk.
+
+##### Note
+
+A `.cpp` file is a form of local scope.
+There is little difference in the opportunities for name clashes in an N-line `.cpp` containing a `using namespace X`,
+an N-line function containing a `using namespace X`,
+and M functions each containing a `using namespace X`with N lines of code in total.
+
+##### Note
+
+[Don't write `using namespace` in a header file](#Rs-using-directive).
 
 ##### Enforcement
 
-???
+Flag multiple `using namespace` directives for different namespaces in a single sourcefile.
 
 ### <a name="Rs-using-directive"></a>SF.7: Don't write `using namespace` in a header file