changelog: add fragment

Author: midoriao

changelog/13879.doc.rst

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+Clarified `pytest/Config.getoption` docstring to make the exceptional behavior of `default` with `skip=True` explicit.
+
+The previous docstring implied:
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+- The `default` parameter is ignored when the option is declared.
+
+But actual behavior is:
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+- The `default` is **not ignored** even if the option is declared, when `skip=True` and the option has `None` value.