Including calling conventions in function declarations - winapi

Question

I am reading about Win32 programming with C/C++ and came across a page which defines the WinMain as:
int WINAPI wWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PWSTR pCmdLine, int nCmdShow);
I understand most of this, except I do not understand where did the part WINAPI come from?
I know that this is a macro for a calling convention. That's not what I am seeking clarity on. My question is not on calling conventions.
When I look at Microsoft's documentation on C++ functions, and I read through optional parts of a function declaration, I don't see any mention of including a calling convention at any place in the function declaration. So where exactly does Microsoft in their documentation speak about including calling conventions in a function declaration?

Answer 1

The portion of Microsoft's documentation that you linked to refers to only standard components of the C++ language. Calling conventions are not part of the C++ specification.
The C++ specification describes how a function can declare its return type and parameters, but it does not define how those values are actually passed between caller and callee. Calling conventions dictate that, and different compilers/platforms implement calling conventions in their own ways. So the C++ specification doesn't describe calling conventions.
In Microsoft's documentation, calling conventions are referred to as Microsoft-Specific Modifiers to the C++ language. Which is technically correct, as any identifier that begins with 1-2 underscores in its name is a vendor-specific extension, and all of the known calling conventions begin with underscores in their names, eg:
__cdecl
__stdcall
__fastcall
__thiscall
__safecall
__vectorcall
__pascal
__fortran
__syscall
etc...
Macros like WINAPI, STDMETHODCALL, etc simply map to a specific calling convention (usually __stdcall, but sometimes __cdecl).
If omitted in a function declaration, the compiler decides which calling convention it wants to use (usually __cdecl).
Compilers from different vendors are not required to implement each others extensions. However, in the case of calling conventions, most compilers at least implement __cdecl and __stdcall, and agree on how they should work, for code portability. But make no mistake, calling conventions are still a vendor-specific extension to the standard language specification.