Adding some information and links for is repr values

Just an enumeration and links to the NativeCall where they are
explained more extensively. Refs #1863 and #1957

doc/Language/traits.pod6

@@ -10,6 +10,30 @@ or changing the implementation. That kind of concept is implemented in Perl 6
 via L<roles|/language/glossary#Roles>; in this context, Perl 6 calls I<traits>
 specifically those that change the internal representation of the class or type.
 
+Traits are used extensively in L<native calls|/language/nativecall> to
+L<specify the
+representation|/language/nativecall#Specifying_the_native_representation> of the
+data structures that are going to be handled by the native functions via the
+C<is repr> suffix; at the same time, C<is native> is used for the routines that
+are actually implemented via native functions. These are the representations
+that can be used:
+
+=item X<CStruct> corresponds to a C<struct> in the C language. It is a composite
+data structure which includes different and heterogeneous lower-level data
+structures; see L<this|/language/nativecall#Structs> for examples and further
+explanations.
+
+=item X<CPPStruct>, similarly, correspond to a C<struct> in C++. However, this
+is Rakudo specific for the time being.
+
+=item X<CPointer> is a pointer in any of these languages. It is a dynamic data
+structure that must be instantiated before being used, can be
+L<used|/language/nativecall#Basic_use_of_Pointers> for classes whose methods are
+also native.
+
+=item X<CUnion> is going to use the same representation as an C<union> in C; see
+L<this|/language/nativecall#CUnions> for an example.
+
 
 
 =end pod