__INTERNAL_fmt: Add description + return with errno -EBADF if FD is lower than FD_stdout (1)

src/console.asm

@@ -101,6 +101,16 @@ printf:
 	mov	rax, FD_stdout
 	jmp	__INTERNAL_fmt
 
+;----- __INTERNAL_fmt(*format[], ...) -----;
+; See printf description above, +
+; 1) Return value can be amount of printed characters or -errno (eg if bad FD was passed, -EBADF is returned)
+; 2) FD has to be passed via RAX (this is clearly an INTERNAL function so passed arguments are slightly different, that is, RAX required for FD)
+; 3) <!> <!> FOR DEVS: When writing an I/O function that calls this wrapper, make sure that:
+;	- That function has function prologue (ie push rbp + mov rbp, rsp)
+;	- __INTERNAL_fmt is called via CALL and not a simple JMP like in printf
+; This is because in such I/O functions, first argument should be file descriptor, and following SYS V ABI, that first argument is passed to RDI
+; However since __INTERNAL_fmt expects FD via RAX, all arguments have to be shifted. Due to this, argument in R9 becomes a stack argument
+; Because of that, __INTERNAL_fmt has to load from stack with a different offset; +24 bytes extra (extra +8 for function call, extra +8 for function prologue and extra +8 to account for possible R9 arg becoming a stack arg)
 __INTERNAL_fmt:
 	%macro load_arg 1
 	cmp	r14, 4
@@ -162,6 +172,11 @@ __INTERNAL_fmt:
 	%endmacro
 
 	; entry:
+	cmp	rax, 0
+	jg	.start_fmt
+	mov	rax, -EBADF
+	ret
+	.start_fmt:
 	push	rbp
 	mov	rbp, rsp
 	push	r12