DOC: Document cache hit function

[larsoner]

Closes #1342

[codecov]

Codecov Report

Base: 94.02% // Head: 93.86% // Decreases project coverage by -0.15% ⚠️

Coverage data is based on head (7438538) compared to base (9239a48).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1343      +/-   ##
==========================================
- Coverage   94.02%   93.86%   -0.16%     
==========================================
  Files          52       52              
  Lines        7292     7292              
==========================================
- Hits         6856     6845      -11     
- Misses        436      447      +11     

Impacted Files	Coverage Δ
joblib/test/testutils.py	50.00% <0.00%> (-50.00%)	⬇️
joblib/_parallel_backends.py	93.01% <0.00%> (-1.84%)	⬇️
joblib/hashing.py	89.47% <0.00%> (-1.76%)	⬇️
joblib/parallel.py	96.03% <0.00%> (-0.27%)	⬇️

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[larsoner]

CI failure appears unrelated

[tomMoral]

LGTM, thx @larsoner