Add note about _CRT_DECLARE_NONSTDC_NAMES

With `/c11` and `/c17` properly defining `__STDC__`, users may encounter that the POSIX functions they are relying on are no longer exposed. This was always the case with `/Za` but is now also visible with the standard C versions.

Author: eldakesh-ms

docs/c-runtime-library/compatibility.md

@@ -20,7 +20,7 @@ The UCRT also implements a large subset of the POSIX.1 (ISO/IEC 9945-1:1996, the
 
 Functions specific to the Microsoft implementation of Visual C++ are found in the vcruntime library.  Many of these functions are for internal use and can't be called by user code. Some are documented for use in debugging and implementation compatibility.
 
-The C++ standard reserves names that begin with an underscore in the global namespace to the implementation. Both the POSIX functions and Microsoft-specific runtime library functions are in the global namespace, but aren't part of the standard C runtime library. That's why the preferred Microsoft implementations of these functions have a leading underscore. For portability, the UCRT also supports the default names, but the Microsoft C++ compiler issues a deprecation warning when code that uses them is compiled. Only the default names are deprecated, not the functions themselves. To suppress the warning, define `_CRT_NONSTDC_NO_WARNINGS` before including any headers in code that uses the original POSIX names.
+The C++ standard reserves names that begin with an underscore in the global namespace to the implementation. Both the POSIX functions and Microsoft-specific runtime library functions are in the global namespace, but aren't part of the standard C runtime library. That's why the preferred Microsoft implementations of these functions have a leading underscore. For portability, the UCRT also supports the default names, but the Microsoft C++ compiler issues a deprecation warning when code that uses them is compiled. Only the default names are deprecated, not the functions themselves. To suppress the warning, define `_CRT_NONSTDC_NO_WARNINGS` before including any headers in code that uses the original POSIX names. Additionally, the C standard forbids additional names in header files, so using `/c11` or `/c17` will not expose these POSIX functions. If these functions are necessary, define `_CRT_DECLARE_NONSTDC_NAMES` to expose them.
 
 Certain functions in the standard C library have a history of unsafe usage, because of misused parameters and unchecked buffers. These functions are often the source of security issues in code. Microsoft created a set of safer versions of these functions that verify parameter usage. They invoke the invalid parameter handler when an issue is detected at runtime.  By default, the Microsoft C++ compiler issues a deprecation warning when a function is used that has a safer variant available. When you compile your code as C++, you can define `_CRT_SECURE_CPP_OVERLOAD_STANDARD_NAMES` as 1 to eliminate most warnings. This macro enables template overloads to call the safer variants while maintaining portable source code. To suppress the warning, define `_CRT_SECURE_NO_WARNINGS` before including any headers in code that uses these functions. For more information, see [Security Features in the CRT](../c-runtime-library/security-features-in-the-crt.md).