docs(12281.feature.rst): add changelog fragment

Author: lovetheguitar

changelog/12281.feature.rst

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+Added support for keyword matching in marker expressions.
+
+Now tests can be selected by marker keyword arguments.
+Supported values are ``int``, (unescaped) ``str``, ``bool`` & ``None``.
+
+See :ref:`marker examples <marker_keyword_expression_example>` for more information.
+
+-- by :user:`lovetheguitar`.