Fix alpha pruning documentation

[xinyuwen2]

• Does this PR have a descriptive title that could go in our release notes?
• Does this PR add any new dependencies?
• Does this PR modify any existing APIs?
• Is the change to the API backwards compatible?
• Should this result in any changes to our documentation, either updating existing docs or adding new ones?

Reference Issues/PRs

What does this implement/fix? Briefly explain your changes.

Corrects comments describing alpha during robust pruning. Higher alpha values make pruning less aggressive and generally retain more edges, producing denser graphs.

Any other comments?

Documentation-only change; no runtime behavior is modified.

[Copilot]

Pull request overview

Updates DiskANN graph pruning documentation to correct how the alpha parameter affects robust pruning behavior, aligning the rustdoc comments with the pruning implementation semantics.

Changes:

• Clarifies that higher alpha generally makes pruning less aggressive and yields denser graphs.
• Updates alpha builder method documentation to match the corrected meaning.
• Refines explanatory text around pruning behavior in the PruneKind rustdoc.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.55%. Comparing base (860cf47) to head (325501a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1351      +/-   ##
==========================================
- Coverage   91.55%   91.55%   -0.01%     
==========================================
  Files         521      521              
  Lines      100347   100347              
==========================================
- Hits        91877    91870       -7     
- Misses       8470     8477       +7     

Flag	Coverage Δ
miri	91.55% <ø> (-0.01%)	⬇️
unittests	91.23% <ø> (-0.01%)	⬇️

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

Files with missing lines	Coverage Δ
diskann/src/graph/config/mod.rs	98.98% <ø> (ø)

... and 3 files with indirect coverage changes

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[Mark Hildebrand (hildebrandmw)]

Thanks! Please wait for Magdalen Dobson Manohar (@magdalendobson) to sign-off.