C++: Unclear wording around using-directive

In the _Namespaces_ section:

> With few exceptions, place code in a namespace. Namespaces should have unique names based on the project name, and possibly its path. Do not use using-directives (e.g., using namespace foo). Do not use inline namespaces. For unnamed namespaces, see Internal Linkage.

This wording gives the impression that the `using` keyword should never be used. However, that would be in direct conflict with an example given later in the section (`using ::foo::Bar;`). [This comment](https://github.com/google/styleguide/pull/204#issuecomment-265744834) by @tituswinters provides a possible explanation, suggesting it is just a bit of unusual terminology:

> Using directives are of the form "using namespace blah;"
> Using declarations are of the form "using myproject::some_symbol;"

I think the wording could be made more clear, for example `Do not use using namespace directives`, if that is the intended meaning.