Update wctomb-s-wctomb-s-l.md

Adding another possible error return.  I'm not sure of the document syntax though.

Author: EddieBreeveld

docs/c-runtime-library/reference/wctomb-s-wctomb-s-l.md

@@ -60,9 +60,11 @@ Error Conditions
 | `NULL` | >0 | `EINVAL` | not modified |
 | any | >`INT_MAX` | `EINVAL` | not modified |
 | any | too small | `EINVAL` | not modified |
-
+ 
 If any of the above error conditions occurs, the invalid parameter handler is invoked, as described in [Parameter validation](../parameter-validation.md). If execution is allowed to continue, `wctomb` returns `EINVAL` and sets `errno` to `EINVAL`.
 
+Return value `EILSEQ` indicates an encoding error has occurred. 
+
 ## Remarks
 
 The **`wctomb_s`** function converts its *`wchar`* argument to the corresponding multibyte character and stores the result at *`mbchar`*. You can call the function from any point in any program.