Accessing C++ flags from Python

[This section of the guide](https://abseil.io/docs/python/guides/flags#how-do-i-access-c-flags-from-python) has been "forthcoming" for some time. Is there any timeline on this?

The definition of `class Flag`, there appear to be some Google internals that "leak" out but are unused.
- [allow_override_cpp](https://github.com/abseil/abseil-py/blob/a0ae31683e6cf3667886c500327f292c893a1740/absl/flags/_flag.py#L49)
- [allow_hide_cpp](https://github.com/abseil/abseil-py/blob/a0ae31683e6cf3667886c500327f292c893a1740/absl/flags/_flag.py#L51)

They seem to be used in [absltest.py](https://github.com/abseil/abseil-py/blob/a0ae31683e6cf3667886c500327f292c893a1740/absl/testing/absltest.py#L228) but is otherwise not well documented.

The issue points to missing documentation for accessing C++ flags from Python in the Abseil guide. Examine the `_flag.py` file to understand the `allow_override_cpp` and `allow_hide_cpp` attributes and their usage in `absltest.py`. Research how C++ flags are integrated and document the process, ensuring the guide section is filled with accurate information.

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