[MRG] fix kreport documentation

[bluegenes]

kreport documentation had a legacy note about abundance-weighting issues that are now solved. This fixes that note and expands the explanation slightly.

I also replaced the example output with results from a real file rather than the test data results that were mostly unclassified.

[codecov]

Codecov Report

Merging #2302 (3c5fe99) into latest (d90c722) will increase coverage by 7.27%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           latest    #2302      +/-   ##
==========================================
+ Coverage   84.76%   92.04%   +7.27%     
==========================================
  Files         131      100      -31     
  Lines       15676    11403    -4273     
  Branches     2252     2252              
==========================================
- Hits        13288    10496    -2792     
+ Misses       2093      612    -1481     
  Partials      295      295              

Flag	Coverage Δ
python	92.04% <ø> (ø)
rust	?

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Impacted Files	Coverage Δ
src/core/tests/test.rs
src/core/src/ffi/index/mod.rs
src/core/src/ffi/utils.rs
src/core/src/ffi/storage.rs
src/core/src/index/sbt/mhbt.rs
src/core/src/index/mod.rs
src/core/src/encodings.rs
src/core/tests/storage.rs
src/core/src/ffi/minhash.rs
src/core/src/ffi/cmd/compute.rs
... and 21 more

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[ctb]

nice!