[clang] [analyzer] Add documentation for std::variant checker (PR #76501)

[via cfe-commits]

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@@ -2097,6 +2097,18 @@ This checker is a part of ``core.StackAddressEscape``, but is temporarily disabl
 
 .. _alpha-core-TestAfterDivZero:
 
+alpha.core.StdVariant (C++)
+"""""""""""""""""""""""""""
+Check if the active value accessed in an ``std::variant`` instance.
+.. code-block:: cpp
+
+ void test() {
+   std::variant <int, char> v = 25;
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whisperity wrote:

```suggestion
   std::variant<int, char> v = 25;
```

https://github.com/llvm/llvm-project/pull/76501