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

@@ -1699,9 +1699,10 @@ def getoption(self, name: str, default=notset, skip: bool = False):
 
         :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 skip: If True, raise pytest.skip if option does not exists
-            or has a None value.
+        :param default: Default value if no option of that name is declared.
+             Note this parameter will be ignored when the option is declared even if the option's value is None.
+        :param skip: If True, raise pytest.skip if option is undeclared or has a None value.
+            Note that even if True, if a default was specified it will be returned instead of a skip.
         """
         name = self._opt2dest.get(name, name)
         try: