[diskann] Add note on the selection of PruneKind in graph::config::Builder.

[Mark Hildebrand (hildebrandmw)]

Add a note to diskann::graph::config::Builder::new() highlighting the conversion from diskann_vector::distance::Metric to diskann::graph::config::PruneKind.

Incorrectly selecting PruneKind can have a profoundly negative impact on recall.

[Copilot]

Pull request overview

Adds developer-facing documentation to diskann::graph::config::Builder::new() to warn that selecting the wrong PruneKind for a given distance metric can significantly harm graph quality/recall, and shows an example of deriving PruneKind from diskann_vector::distance::Metric.

Changes:

• Add a rustdoc “Note” section to Builder::new() explaining that PruneKind selection depends on the distance function.
• Add a rustdoc example showing how to convert Metric into PruneKind when constructing a config::Builder.

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[Codecov Comments Bot (codecov-commenter)]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.31%. Comparing base (f3fcaae) to head (1a6ae7b).

Additional details and impacted files

@@ Coverage Diff @@## main #734 +/- ##
==========================================
+ Coverage 89.01% 90.31% +1.30% 
==========================================
Files 428 428 Lines 78294 78294 ==========================================
+ Hits 69692 70710 +1018 + Misses 8602 7584 -1018 

Files with missing lines	Coverage Δ
diskann/src/graph/config/mod.rs	98.07% <ø> (ø)

... and 39 files with indirect coverage changes

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