Modify rule S3470: Clarify that we allow std class specialization

rules/S3470/cfamily/rule.adoc

@@ -1,6 +1,6 @@
 == Why is this an issue?
 
-It may seem tidy to add your new declarations to the ``++std++`` or ``++posix++`` namespaces, but doing so results in undefined behavior. The {cpp}14 Standard, [namespace.std] (ISO/IEC 14882-2014 §17.6.4.2.1), paragraphs 1 and 2 states:
+It may seem tidy to add your new declarations to the `std` or `posix` namespaces, but doing so results in undefined behavior. The {cpp}14 Standard, [namespace.std] (ISO/IEC 14882-2014 §17.6.4.2.1), paragraphs 1 and 2 states:
 
 
 ____
@@ -12,17 +12,20 @@ ____
 ** an explicit or partial specialization of any member class template of a standard library class or class template.
 ____
 
-In addition to restricting extensions to the ``++std++`` namespace, the {cpp}14 Standard goes on in §17.6.4.2.2 to say:
+In addition to restricting extensions to the `std` namespace, the {cpp}14 Standard goes on in §17.6.4.2.2 to say:
 
 ____
 
 . The behavior of a {cpp} program is undefined if it adds declarations or definitions to namespace posix or to a namespace within namespace posix unless otherwise specified. The namespace posix is reserved for use by ISO/IEC 9945 and other POSIX standards.
 ____
 
-You may think that it's legitimate to reopen ``++std++`` to define a version of extension points (``++std::swap++``, ``++std::hash++``...) that work with your types, but it's not necessary:  If you call these extension points according to the correct pattern, the user-defined version will be found too.
+However, the standard allows specializing standard class templates in namespace `std`. In that case, the specialization must respect the requirement of the original template and has to be for a "program-defined type" (a type that is specific to the program, by opposition to a type from the standard).
 
+You may therefore think that it's legitimate to reopen `std` to define a version of extension points (``++std::swap++``, ``++std::hash++``...) that work with your types, but it's not necessary:  If you call these extension points according to the correct pattern, the user-defined version will be found too.
 
-This rule raises an issue for any modification of the standard ``++std++`` and ``++posix++`` namespaces.
+The only extension points for which the specialization is the recommended approach are ``++std::out_ptr++`` and ``++std::inout_ptr++``.
+
+This rule raises an issue for any modification of the standard `std` and `posix` namespaces that is not a template specialization.
 
 
 === Noncompliant code example
@@ -53,11 +56,6 @@ namespace MyNamespace {
 ----
 
 
-=== Exceptions
-
-A namespace fragment that only contains template specializations or explicit instantiations is ignored by this rule.
-
-
 == Resources
 
 * https://wiki.sei.cmu.edu/confluence/x/Xnw-BQ[CERT, DCL58-CPP.] - Do not modify the standard namespaces