Update wcstombs-wcstombs-l.md (#4478)

* Update wcstombs-wcstombs-l.md

Starting in Windows 10 version 1803 (10.0.17134.0), the Universal C Runtime supports using a UTF-8 code page. Two bytes for every wide character may not be enough,best to use wcstombs (NULL, wcstr, 0) to get the size correctly.

* Update wcstombs-wcstombs-l.md

Add code escape, fix formatting of the function call, etc.

* Update wcstombs-wcstombs-l.md

Added link for more info about UTF-8

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Co-authored-by: Tyler Whitney <[email protected]>

Author: fjh1997

docs/c-runtime-library/reference/wcstombs-wcstombs-l.md

@@ -64,6 +64,8 @@ If **`wcstombs`** successfully converts the multibyte string, it returns the num
 
 The **`wcstombs`** function converts the wide-character string pointed to by *`wcstr`* to the corresponding multibyte characters and stores the results in the *`mbstr`* array. The *`count`* parameter indicates the maximum number of bytes that can be stored in the multibyte output string (that is, the size of *`mbstr`*). In general, it isn't known how many bytes will be required when converting a wide-character string. Some wide characters will require only a single byte in the output string; others require 2 bytes. If there are 2 bytes in the multibyte output string for every wide character in the input string (including the wide character `NULL`), the result is guaranteed to fit.
 
+Starting in Windows 10 version 1803 (10.0.17134.0), the Universal C Runtime supports using a UTF-8 code page. Use `wcstombs(NULL, wcstr, 0)` to get the correct size that you'll need for the conversion because assuming that you'll need two bytes for every wide character may not be enough. For more information about UTF-8 support, see [UTF-8 support](setlocale-wsetlocale.md)
+
 If **`wcstombs`** encounters the wide-character `NULL` character (L'\0') either before or when *`count`* occurs, it converts it to an 8-bit 0 and stops. Thus, the multibyte character string at *`mbstr`* is null-terminated only if **`wcstombs`** encounters a wide-character `NULL` character during conversion. If the sequences pointed to by *`wcstr`* and *`mbstr`* overlap, the behavior of **`wcstombs`** is undefined.
 
 If the *`mbstr`* argument is `NULL`, **`wcstombs`** returns the required size in bytes of the destination string.