Update vscprintf_p and vscwprintf_p

Author: majobara-ms

docs/c-runtime-library/reference/vscprintf-p-vscprintf-p-l-vscwprintf-p-vscwprintf-p-l.md

@@ -59,6 +59,9 @@ These functions differ from **`_vscprintf`** and **`_vscwprintf`** only in that
 
 The versions of these functions with the **`_l`** suffix are identical except that they use the locale parameter passed in instead of the current thread locale.
 
+Return Value is the size of the formatted data. If the function takes a `char` buffer, the size is in bytes. If the function takes a `wchar_t` buffer, the size specifies the number of 16-bit words.
+Characters refer to `char` characters for functions that take a `char` buffer, and to `wchar_t` characters for functions that take a `wchar_t` buffer.
+
 If *`format`* is a null pointer, the invalid parameter handler is invoked, as described in [Parameter validation](../parameter-validation.md). If execution is allowed to continue, the functions return -1 and set `errno` to `EINVAL`.
 
 > [!IMPORTANT]