add a fix for github 4688 - __fastcall parameter passing clarification

Author: TylerMSFT

docs/cpp/fastcall.md

@@ -1,10 +1,9 @@
 ---
 description: "Learn more about: __fastcall"
 title: "__fastcall"
-ms.date: "12/17/2018" 
+ms.date: 09/14/2023
 f1_keywords: ["__fastcall_cpp", "__fastcall", "_fastcall"]
 helpviewer_keywords: ["__fastcall keyword [C++]"]
-ms.assetid: bb5b9c8a-dfad-450c-9119-0ac2bc59544f
 ---
 # __fastcall
 
@@ -14,10 +13,12 @@ The **`__fastcall`** calling convention specifies that arguments to functions ar
 
 |Element|Implementation|
 |-------------|--------------------|
-|Argument-passing order|The first two DWORD or smaller arguments that are found in the argument list from left to right are passed in ECX and EDX registers; all other arguments are passed on the stack from right to left.|
+|Argument-passing order|The first two `DWORD` or smaller arguments that are found in the argument list from left to right are passed in ECX and EDX registers; all other arguments are passed on the stack from right to left.|
 |Stack-maintenance responsibility|Called function pops the arguments from the stack.|
 |Name-decoration convention|At sign (\@) is prefixed to names; an at sign followed by the number of bytes (in decimal) in the parameter list is suffixed to names.|
 |Case-translation convention|No case translation performed.|
+|Classes, structs, and unions|Treated as "multibyte" types (regardless of size) and passed on the stack. |
+|Enums and enum classes | Passed by register if their underlying type would be passed by register. For example, if the underlying type is `int` or `unsigned int` of size 8, 16, or 32 bits. |
 
 > [!NOTE]
 > Future compiler versions may use different registers to store parameters.
@@ -46,7 +47,7 @@ is equivalent to this:
 void __fastcall CMyClass::mymethod() { return; }
 ```
 
-For compatibility with previous versions, **_fastcall** is a synonym for **`__fastcall`** unless compiler option [/Za \(Disable language extensions)](../build/reference/za-ze-disable-language-extensions.md) is specified.
+For compatibility with previous versions, **`_fastcall`** is a synonym for **`__fastcall`** unless compiler option [/Za \(Disable language extensions)](../build/reference/za-ze-disable-language-extensions.md) is specified.
 
 ## Example