stdlib: document return values of mkstemp correctly
* ext/posix/stdlib.c (Pmkstemp): filename is the second return value
from a successful call.
Reported by: Bernd Eggink
@@ -141,7 +141,7 @@ Create a unique temporary file.
@function mkstemp
@string templ pattern that ends in six 'X' characters
@treturn[1] int open file descriptor
-@treturn[2] string path to file, if successful
+@treturn[1] string path to file, if successful
@return[2] nil
@treturn[2] string error message
@treturn[2] int errnum
