fix Issue 23698 - ImportC: __stdcall is not documented as supported MSVC/DMC extensions (#3580)

Author: WalterBright

spec/importc.dd

@@ -626,9 +626,17 @@ $(H2 $(LNAME2 gnu-clang-extensions, Gnu and Clang Extensions))
 
 $(H2 $(LNAME2 visualc-extensions, Visual C Extensions))
 
+    $(P All the $(RELATIVE_LINK2 digital-mars-extensions, Digital Mars C Extensions).)
 
 $(H2 $(LNAME2 digital-mars-extensions, Digital Mars C Extensions))
 
+    $(H3 $(LNAME2 __stdcall, `__stdcall` Function Calling Convention))
+
+    $(P This sets the calling convention for a function to the Windows API calling convention.)
+
+    ---
+    int __stdcall foo(int x);
+    ---
 
 $(H2 $(LNAME2 d-side, ImportC from D's Point of View))