clarify difference between declaration and definition (#3629)

Author: WalterBright

spec/attribute.dd

@@ -429,12 +429,22 @@ $(H3 $(LNAME2 export, $(D export) Attribute))
         or DLL.)
 
         $(P $(D export) applied to the definition of a symbol will export it. $(D export) applied to
-        a declaration of a symbol will import it.)
+        a declaration of a symbol will import it.
+        A variable is a definition unless `extern` is applied to it.)
 
         ---
         export int x = 3;    // definition, exporting `x`
         export int y;        // definition, exporting `y`
         export extern int z; // declaration, importing `z`
+
+        export __gshared h = 3;        // definition, exporting `h`
+        export __gshared i;            // definition, exporting `i`
+        export extern __gshared int j; // declaration, importing `j`
+        ---
+
+        $(P A function with a body is a definition, without a body is a declaration.)
+
+        ---
         export void f() { }  // definition, exporting `f`
         export void g();     // declaration, importing `g`
         ---