docs: Update JAX backend documentation on training support

[njzjz]

Summary by CodeRabbit

• Documentation
  • Updated backend documentation by refining content clarity.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 070eb158-32ce-40fb-8299-e5dc46d726a8

📥 Commits

Reviewing files that changed from the base of the PR and between ccaa551 and b507784.

📒 Files selected for processing (1)

• doc/backend.md

💤 Files with no reviewable changes (1)

• doc/backend.md

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📝 Walkthrough

Walkthrough

This pull request removes a single sentence from backend documentation that stated the backend is actively developed and lacks training support. No other backend or format details are altered.

Changes

Documentation Update

Layer / File(s)	Summary
Backend documentation edit doc/backend.md	Sentence about missing training support is deleted from the backend documentation.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: updating JAX backend documentation regarding training support.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[Copilot]

Copilot reviewed 1 out of 1 changed files in this pull request and generated 1 comment.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.34%. Comparing base (d67e75b) to head (b507784).
⚠️ Report is 19 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5482      +/-   ##
==========================================
- Coverage   82.46%   81.34%   -1.13%     
==========================================
  Files         830      868      +38     
  Lines       88414    96358    +7944     
  Branches     4232     4236       +4     
==========================================
+ Hits        72913    78383    +5470     
- Misses      14215    16675    +2460     
- Partials     1286     1300      +14     

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