First attempt at documenting nativetypes

I have moved the table to this page and split it in two, eliminating
nonspec types. It will clarify what you can use in native call (those
declared with nativesize) and which ones you can't. Refs #1512.

doc/Language/nativetypes.pod6

@@ -4,21 +4,34 @@
 
 =SUBTITLE Using the types the compiler and hardware make available to you.
 
-=head1 Integer types and their behavior
-
-TBD
-
-=head1 Strings and arrays
-
-TBD
-
-=head1 C<structs>
-
-TBD
-
-=head1 Native representation
-
-TBD
+Some simple types in Perl 6 have a I<native> representation, indicating that they will use the C language representation provided by the compiler, operating system and machine. These are the four native types available:
+
+X<|int>X<|uint>X<|num>X<|str>
+=begin table
+    int            Equivalent to Int
+    uint           Equivalent to Int with the unsigned trait
+    num            Equivalent to Num
+    str            Equivalent to Str
+=end table
+
+However, these types do not necessarily have the size that is required by the
+NativeCall interface (e.g., Perl 6's C<int> can be 8 bytes but C's C<int> is
+only 4 bytes). Instead, the types below will have to be used instead of the
+types C<int> or C<num> listed above.
+
+X<|int8>X<|int16>X<|int32>X<|int64>X<|uint8>X<|uint16>X<|uint32>X<|uint64>X<|num32>X<|num64>
+=begin table
+    int8           (int8_t in C, also used for char)
+    int16          (int16_t in C, also used for short)
+    int32          (int32_t in C, also used for int)
+    int64          (int64_t in C)
+    uint8          (uint8_t in C, also used for unsigned char)
+    uint16         (uint16_t in C, also used for unsigned short)
+    uint32         (uint32_t in C, also used for unsigned int)
+    uint64         (uint64_t in C)
+    num32          (float in C)
+    num64          (double in C)
+=end table
 
 
 =end pod