lib/vsprintf: Improve vsprintf + sprintf function comments

Clarify that the return values of vsprintf() and sprintf() exclude the
trailing NUL character.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20251103090913.2066-2-thorsten.blum@linux.dev
Signed-off-by: Petr Mladek <pmladek@suse.com>

Author: toblux

lib/vsprintf.c

@@ -3054,8 +3054,8 @@ EXPORT_SYMBOL(scnprintf);
  * @fmt: The format string to use
  * @args: Arguments for the format string
  *
- * The function returns the number of characters written
- * into @buf. Use vsnprintf() or vscnprintf() in order to avoid
+ * The return value is the number of characters written into @buf not including
+ * the trailing '\0'. Use vsnprintf() or vscnprintf() in order to avoid
  * buffer overflows.
  *
  * If you're not already dealing with a va_list consider using sprintf().
@@ -3074,8 +3074,8 @@ EXPORT_SYMBOL(vsprintf);
  * @fmt: The format string to use
  * @...: Arguments for the format string
  *
- * The function returns the number of characters written
- * into @buf. Use snprintf() or scnprintf() in order to avoid
+ * The return value is the number of characters written into @buf not including
+ * the trailing '\0'. Use snprintf() or scnprintf() in order to avoid
  * buffer overflows.
  *
  * See the vsnprintf() documentation for format string extensions over C99.