Add description for new warning C26826

Author: dmitrykobets-msft

docs/code-quality/c26826.md

@@ -0,0 +1,44 @@
+---
+title: C26826
+description: "Reference for Microsoft C++ Code Analysis warning C26826 in Visual Studio."
+ms.date: 10/25/2021
+f1_keywords: ["C26826"]
+helpviewer_keywords: ["C26826"]
+---
+# C26826
+
+> Don't use C-style variable arguments (f.55).
+
+For more information, see [F.55: Don't use `va_arg` arguments](https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#f55-dont-use-va_arg-arguments) in the C++ Core Guidelines.
+
+This check warns on all usages of `va_list`, `va_start`, `va_arg`, and `va_end`, discouraging the use of C-style variable arguments. C-style variable arguments are unsafe because they require the programmer to assume that the arguments are all passed / read with the correct types.
+
+## Example
+
+```cpp
+int sum(int n, ...) {
+    va_list l; // C26826 Don't use C-style variable arguments
+    va_start(l, n); // C26826 Don't use C-style variable arguments
+
+    int s = 0;
+    for (int i = 0; i < n; ++i) {
+        // BAD, assumes the variable arguments will be passed as ints
+        s += va_arg(l, int); // C26826 Don't use C-style variable arguments
+    }
+
+    va_end(l); // C26826 Don't use C-style variable arguments
+    return s;
+}
+
+int main() {
+    sum(2, 1, 2, 3); // ok
+    sum(2, 1.5, 3.14159, 2.71828); // BAD, undefined
+}
+```
+
+Alternatives to C-style variable arguments include:
+- function overloading
+- variadic templates
+- `std::variant` arguments
+- `std::initializer_list`
+