docs(nnx): fix training API reference inconsistencies

[greekera1000]

What does this PR do?

Fixes a few inconsistencies in the flax.nnx.training API reference (part of #5161):

• EMA.apply_to() is public and used in EMA's own docstring examples, but wasn't rendered in the reference — added it to :members:.
• Accuracy listed only update while the sibling metrics list __init__, reset, update, compute — aligned it (this also surfaces the threshold constructor doc).
• Statistics (returned by Welford.compute()) had no docstring — added one.

Scope: docs + docstrings only, no behavioral changes. All training module doctests pass locally.

Relates to #5161.

Checklist

• This PR fixes a minor issue (e.g.: typo or small bug) or improves the docs (you can dismiss the other checks if that's the case).
• This change is discussed in a Github issue: [docs]: Improve docs for NNX API Reference #5161
• The documentation and docstrings adhere to the documentation guidelines.
• This change includes necessary high-coverage tests. (No quality testing = no merge!)

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[greekera1000]

@googlebot I signed it!