[MRG] comments update for ZipFileLinearIndex

[ctb]

argh, forgot to push this change before merging #1349. So now it's a new PR.

This PR adds a comment to a confusing section of the signature loading code in ZipFileLinearIndex.signatures(...).

[codecov]

Codecov Report

Merging #1442 (7adf978) into latest (beb3d59) will increase coverage by 5.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           latest    #1442      +/-   ##
==========================================
+ Coverage   89.44%   94.59%   +5.14%     
==========================================
  Files         123       96      -27     
  Lines       19105    15491    -3614     
  Branches     1471     1471              
==========================================
- Hits        17089    14654    -2435     
+ Misses       1784      605    -1179     
  Partials      232      232              

Flag	Coverage Δ
python	94.59% <ø> (ø)
rust	?

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Impacted Files	Coverage Δ
src/sourmash/index.py	92.10% <ø> (ø)
src/core/src/ffi/minhash.rs
src/core/src/ffi/hyperloglog.rs
src/core/src/wasm.rs
src/core/src/cmd.rs
src/core/src/ffi/signature.rs
src/core/src/ffi/cmd/compute.rs
src/core/tests/test.rs
src/core/src/ffi/utils.rs
src/core/src/index/bigsi.rs
... and 18 more

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