[BUG] Series accessors aren't being doctested.

**Describe the bug**
It appears that the special type accessors for `Series.str` (`StringMethods`) are not being doctested. See https://github.com/rapidsai/cudf/pull/11575#discussion_r953925683. I suspect this is also true for list and struct accessors but have not verified that.

**Expected behavior**
Doctests should run for type-accessors inheriting from `ColumnMethods` like `Series.str.findall`.

This will probably require some updates to [test_doctests.py](https://github.com/rapidsai/cudf/blob/main/python/cudf/cudf/tests/test_doctests.py) to ensure that these accessor methods are found and tested. It may require a special set of tests explicitly designed for finding and testing all `ColumnMethods` objects.

Start by reading python/cudf/cudf/tests/test_doctests.py and checking how Series accessors such as Series.str.findall are discovered. Verify whether list and struct accessors inheriting from ColumnMethods are also omitted. Done means doctests run for these accessor methods, with tests covering the discovery behavior.

Written by the indexing model from the issue text.