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

[Balazs Benics via cfe-commits]

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@@ -2097,6 +2097,21 @@ This checker is a part of ``core.StackAddressEscape``, but is temporarily disabl
 
 .. _alpha-core-TestAfterDivZero:
 
+alpha.core.StdVariant (C++)
+"""""""""""""""""""""""""""
+Check if a value of active type is retrieved from an ``std::variant`` instance with ``std::get``.
+In case of bad variant type access (the accessed type differs from the active type)
+a warning is emitted. Currently this checker does not take exception handling into account.
+
+.. code-block:: cpp
+
+ void test() {
+   std::variant <int, char> v = 25;
+   char c = stg::get<char>(v); // warn
----------------
steakhal wrote:

```suggestion
   char c = stg::get<char>(v); // warn: "int" is the active alternative
```

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