fix HardMaxCacheSize documentation

[hanshuhuang]

As #292 has pointed out, the documentation has misleading message for HardMaxCacheSize.

To solve it, this PR modifies the HardMaxCacheSize comment.

[codecov-commenter]

Codecov Report

Merging #298 (6ee2fde) into master (74e2db1) will decrease coverage by 0.52%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #298      +/-   ##
==========================================
- Coverage   88.97%   88.45%   -0.53%     
==========================================
  Files          15       15              
  Lines         762      762              
==========================================
- Hits          678      674       -4     
- Misses         70       73       +3     
- Partials       14       15       +1     

Impacted Files	Coverage Δ
config.go	100.00% <ø> (ø)
shard.go	86.90% <0.00%> (-1.59%)	⬇️

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