Behavior summary update.

Author: nikita-leontiev

docs/c-runtime-library/reference/vsnprintf-vsnprintf-vsnprintf-l-vsnwprintf-vsnwprintf-l.md

@@ -155,7 +155,8 @@ For the following table:
 | Encoding error during formatting | If processing string specifier `s`, `S`, or `Z`, format specification processing stops. | -1 | `EILSEQ` (42) | No |
 | Encoding error during formatting | If processing character specifier `c` or `C`, the invalid character is skipped. The number of characters written isn't incremented for the skipped character, nor is any data written for it. Processing the format specification continues after skipping the specifier with the encoding error. | The number of characters written, not including the terminating `NULL`. | `EILSEQ` (42) | No |
 | `buffer == NULL` and `count != 0` | If execution continues after invalid parameter handler executes, sets `errno` and returns a negative value.| -1 | `EINVAL` (22) | Yes |
-| `count == 0` | No data is written | The number of characters that would have been written, not including the terminating `NULL`. You can use this result to allocate sufficient buffer space for the string and a terminating `NULL`, and then call the function again to fill the buffer. | N/A | No |
+| `buffer == NULL` and `count == 0` | No data is written | The number of characters that would have been written, not including the terminating `NULL`. You can use this result to allocate sufficient buffer space for the string and a terminating `NULL`, and then call the function again to fill the buffer. | N/A | No |
+| `count == 0` | No data is written | -1 | `ERANGE` (34) | No |
 | `count < 0`| Unsafe: the value is treated as unsigned, likely creating a large value that results in overwriting the memory that follows the buffer. | The number of characters written. | N/A | No |
 | `count < sizeOfBuffer` and `len <= count` | All of the data is written and a terminating `NULL` is appended. | The number of characters written, not including the terminating `NULL`. | N/A | No |
 | `count < sizeOfBuffer` and `len > count` | The first *`count-1`* characters are written followed by a null-terminator. | The number of characters that would have been written had `count` matched the number of characters to output, not including the null-terminator. | N/A | No |