docs: clearify HummockClient's location

[skyzh]

What's changed and what's your intention?

As title

Checklist

- [ ] I have written necessary docs and comments
- [ ] I have added necessary unit tests and integration tests

Refer to a related PR or issue link (optional)

[codecov]

Codecov Report

Merging #2544 (e6b2ddd) into main (db3c72d) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #2544   +/-   ##
=======================================
  Coverage   71.40%   71.40%           
=======================================
  Files         689      689           
  Lines       88528    88555   +27     
=======================================
+ Hits        63209    63231   +22     
- Misses      25319    25324    +5     

Flag	Coverage Δ
rust	71.40% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/storage/src/hummock/conflict_detector.rs	96.01% <0.00%> (-2.86%)	⬇️
src/storage/src/hummock/local_version_manager.rs	89.92% <0.00%> (+0.03%)	⬆️
src/common/src/types/ordered_float.rs	24.90% <0.00%> (+0.19%)	⬆️
...rc/hummock/shared_buffer/shared_buffer_uploader.rs	82.24% <0.00%> (+0.33%)	⬆️

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