Clarify vswprintf return values for count zero

behavior is a little different for count = 0 case.

Author: TylerMSFT

docs/c-runtime-library/reference/vsprintf-vsprintf-l-vswprintf-vswprintf-l-vswprintf-l.md

@@ -95,7 +95,9 @@ The locale to use.
 
 **`vsprintf`** and **`vswprintf`** return the number of characters written, not including the terminating `NULL` character, or a negative value if an output error occurs. If *`buffer`* or *`format`* is a `NULL` pointer, these functions invoke the invalid parameter handler, as described in [Parameter validation](../parameter-validation.md). If execution is allowed to continue, these functions return -1 and set `errno` to `EINVAL`.
 
-If *`buffer`* is a `NULL` pointer and *`count`* is zero, **`vswprintf`** and **`_vswprintf_l`** return the number of characters that would have been written not including the terminating NULL. If only *`count`* is zero, behavior is unpredictable.
+If *`buffer`* is a `NULL` pointer and *`count`* is zero, **`vswprintf`** and **`_vswprintf_l`** return the number of characters that would have been written not including the terminating NULL.
+
+If *`buffer`* is valid and *`count`* is zero, **`vswprintf`** and **`_vswprintf_l`** return -1.
 
 For information on these and other error codes, see [`errno`, `_doserrno`, `_sys_errlist`, and `_sys_nerr`](../errno-doserrno-sys-errlist-and-sys-nerr.md).