Add (Docs) -> Added doc for newly added callconv attribute

doc/docs.md

@@ -5978,10 +5978,15 @@ fn custom_allocations() {
 struct C.Foo {
 }
 
-// Used in Win32 API code when you need to pass callback function
-[windows_stdcall]
+// Used to add a custom calling convention to a function, available calling convention: stdcall, fastcall and cdecl.
+// This list aslo apply for type aliases (see below).
+[callconv: "stdcall"]
 fn C.DefWindowProc(hwnd int, msg int, lparam int, wparam int)
 
+// Used to add a custom calling convention to a function type aliases.
+[callconv: "fastcall"]
+type FastFn = fn (int) bool
+
 // Windows only:
 // If a default graphics library is imported (ex. gg, ui), then the graphical window takes
 // priority and no console window is created, effectively disabling println() statements.