[clang-tools-extra] [clang-tidy] Rename 'cert-dcl50-cpp' to 'modernize-avoid-variadic-functions' (PR #157737)

[Baranov Victor via cfe-commits]

================
@@ -1,10 +1,13 @@
 .. title:: clang-tidy - cert-dcl50-cpp
+.. meta::
+   :http-equiv=refresh: 5;URL=../modernize/avoid-variadic-functions.html
 
 cert-dcl50-cpp
 ==============
 
-This check flags all function definitions (but not declarations) of C-style
-variadic functions.
+The cert-dcl50-cpp check is an alias, please see
----------------
vbvictor wrote:

```suggestion
The `cert-dcl50-cpp` check is an alias, please see
```

https://github.com/llvm/llvm-project/pull/157737