First call to 'sphinx-build -b doctest' does not test generated documents.

As the title says, you would have to execute `sphinx-build -b doctest` (or `make doctest`) twice if you want to run all your tests starting from a fresh environment (e.g. with a CI pipeline):
```zsh
% rm -r build api/autogen
% sphinx-build -b doctest . build|grep 'tests$'
425 tests
0 failures in tests
% sphinx-build -b doctest . build|grep 'tests$'
740 tests
0 failures in tests
```

Reproduce the clean-environment case with `rm -r build api/autogen` and the first `sphinx-build -b doctest . build` or `make doctest` run. Trace the Sphinx doctest build and generated `api/autogen` output; done means one fresh invocation discovers and runs all generated-document tests, matching the second run's 740 tests.

Written by the indexing model from the issue text.