GeezerVison

The docstring for the public httpcore.stream() function tells users how to
read a streamed response body:

If you want to access the response body you should either use
content = response.read(), or for chunk in response.iter_content().

But Response has no iter_content() method — the streaming iterator is
iter_stream() (and aiter_stream() for async). Running the documented
for chunk in response.iter_content() raises AttributeError.

The rest of the project already uses the correct name (docs/quickstart.md
documents for chunk in response.iter_stream():, and the tests use
response.iter_stream()). This one docstring drifted because doctests are
not run in CI.

This corrects the docstring to iter_stream() and adds a small regression
test pinning the documented streaming method names, so the documented API
can't silently drift to a nonexistent method again.

No library behaviour changes — docs-only correctness plus a guard test.