Clarify documentation for OBSERVABLE_INCLUDE.

[viathor]

No description provided.

[viathor]

Had to make a few updates to make doc generation work apparently necessary due to python upgrade. At least __annotate_func__ and __annotations_cache__ are attributes introduced in python 3.14. I don't know for sure, because I don't have python 3.14 installed, so had limited ability to investigate.

[Strilanc]

I'm enjoying seeing all the ways to not end up with the exact correct docs.

Do

bazel build :stim_dev_wheel && pip uninstall stim --yes && pip install bazel-bin/stim-0.0.dev0-py3-none-any.whl

instead of pip install .. The bazel method is much faster. I should probably update the doc generation to remove the weird multiplexing stuff that's currently leaking through from yours.