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

[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-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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