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

Add a short documentation for `StdVariantChecker`.

---------

Co-authored-by: Balazs Benics <benicsbalazs@gmail.com>
Co-authored-by: whisperity <whisperity@gmail.com>
Co-authored-by: DonatNagyE <donat.nagy@ericsson.com>

clang/docs/ReleaseNotes.rst

@@ -1072,6 +1072,7 @@ New features
 
 - Added a new experimental checker ``alpha.core.StdVariant`` to detect variant
   accesses via wrong alternatives.
+  `Documentation <https://clang.llvm.org/docs/analyzer/checkers.html#alpha-core-stdvariant-c>`__.
   (`#66481 <https://github.com/llvm/llvm-project/pull/66481>`_)
 
 - Added a new experimental checker ``alpha.cplusplus.ArrayDelete`` to detect

clang/docs/analyzer/checkers.rst

@@ -2095,6 +2095,21 @@ This checker is a part of ``core.StackAddressEscape``, but is temporarily disabl
                  //       returned block
  }
 
+.. _alpha-core-StdVariant:
+
+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: "int" is the active alternative
+ }
+
 .. _alpha-core-TestAfterDivZero:
 
 alpha.core.TestAfterDivZero (C)