Add new_thread_unsafe_stream to the devices and streams docs

[ayaangazali]

Proposed changes

new_thread_unsafe_stream is public and works, and it has a full docstring that cross references :func:new_stream, but it never made it into the API reference. Its two siblings new_stream and new_thread_local_stream are both listed in devices_and_streams.rst, so right now the only way to find this one is to already know it exists.

import mlx.core as mx
s = mx.new_thread_unsafe_stream(mx.cpu)
mx.add(mx.array(1), mx.array(2), stream=s)   # array(3, dtype=int32)

Looks like it was just missed when the API was added in #3578. This adds the one autosummary entry, next to the other two stream constructors.

While I was checking, I also compared every public callable in mlx.core against the whole of docs/src and this was the only properly bound function absent from the docs. The remaining unlisted names (pow, matrix_norm, cumulative_sum, acos and friends) are plain aliases whose docstrings render under the aliased name, so listing those would show a signature like power(...) on a page called pow. I left them alone since that looks deliberate, but happy to revisit if you would rather they were documented too.

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)

(docs only, one line)

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for transparency: freshman contributor, i use Claude Code to help me sweep for gaps like this, but i verified the function actually runs, read its docstring, and diffed the full public API against the docs myself before opening. the alias question above is a genuine one, not rhetorical, so tell me if you want those listed as well.