docs: clarify method vs transformer in sc.pp.neighbors

[CuiweiG]

Problem

The sc.pp.neighbors docstring bundles the kNN search and the
connectivity kernel into a single lead sentence attributing both to
UMAP. The parameters controlling the two stages are independent:
transformer selects the kNN search backend, method selects the
connectivity kernel. The lead paragraph and the method /
transformer parameter descriptions obscure this.

Change

• Rewrite the lead paragraph to describe the two stages separately.
• Add a sentence to the method parameter description stating that
  the choice is independent of the kNN search backend.
• Replace "Approximate kNN search implementation" with "kNN search
  backend" in the transformer description. The original wording
  contradicted the same block, which already stated that exact kNN
  is used for small data.

No signature, default, or behavior change.

Verification

• sphinx-build -W --keep-going produces 0 new warnings; all four
  citations (McInnes2018, Coifman2005, Haghverdi2016, Levine2015)
  resolve; both :class: cross-references resolve.
• pre-commit run --all-files: 15/15 hooks pass.
• pytest tests/test_neighbors.py tests/test_neighbors_common.py tests/test_neighbors_key_added.py: 36 passed.

Closes #277.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.60%. Comparing base (9909599) to head (5c394fc).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4079   +/-   ##
=======================================
  Coverage   78.60%   78.60%           
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  Files         118      118           
  Lines       12755    12755           
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  Hits        10026    10026           
  Misses       2729     2729           

Flag	Coverage Δ
hatch-test.low-vers	77.90% <ø> (ø)
hatch-test.pre	78.48% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/scanpy/neighbors/__init__.py	82.04% <ø> (ø)

[flying-sheep]

Thanks, looks good! You marked this as draft, do you plan to make more changes?

[CuiweiG]

Thanks! Nothing more to change — I had it in draft pending feedback. Switching to ready now.