Add unsupported features in NOTES-C99.md: complex.h and variable length array

Resolves: https://github.com/openssl/openssl/issues/28598

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
(Merged from https://github.com/openssl/openssl/pull/28602)

NOTES-C99.md

@@ -20,3 +20,16 @@ The list here is going to be updated by features we either
 The list of C-99 features we don't support in OpenSSL project follows:
 
    - do not use `//` for comments, stick to `/* ... */`
+
+   - do not use `<complex.h>`. MSVC doesn't quite implement it to standard.
+
+   - do not use variable length arrays, i.e. arrays where the size is
+     determined by another variable.  MSVC doesn't implement it at all.
+     For clarity, this is an example of such an array:
+
+     ``` C
+     int fun(size_t n)
+     {
+         char s[n]; /* variable size array */
+         ...
+     ```