Fix ambiguous docstring of pytest.Config

For full context see issue pytest-dev#10558

changelog/10558.doc.rst

@@ -0,0 +1 @@
+Fix ambiguous docstring of `pytest.Config.getoption`.

src/_pytest/config/__init__.py

@@ -1695,13 +1695,16 @@ def _get_override_ini_value(self, name: str) -> str | None:
         return value
 
     def getoption(self, name: str, default=notset, skip: bool = False):
-        """Return command line option value.
+        """Return command line option value if exists
+        An option "exists" if the parser has been taught to expect it using parser.addoption in pytest_addoption hook.
 
         :param name: Name of the option.  You may also specify
             the literal ``--OPT`` option instead of the "dest" option name.
-        :param default: Default value if no option of that name exists.
+        :param default: Default value if no option of that name exists (see note on option existence).
+             If an option exists then this parameter will be ignored even if the option's value is None.
         :param skip: If True, raise pytest.skip if option does not exists
-            or has a None value.
+            or has a None value. In case the option name doesn't exist,
+            but a default param was passed, then the default will be returned instead of a skip.
         """
         name = self._opt2dest.get(name, name)
         try: