docs: add missing docstrings for core

[Yongxuanzhang]

Improve the docstring coverage in Jina core to 100% 🥳 .

[codecov]

Codecov Report

Merging #2040 (ac7f7de) into master (1f4ded9) will decrease coverage by 15.67%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #2040       +/-   ##
===========================================
- Coverage   88.75%   73.08%   -15.68%     
===========================================
  Files         200      181       -19     
  Lines       10933    10372      -561     
===========================================
- Hits         9704     7580     -2124     
- Misses       1229     2792     +1563     

Flag	Coverage Δ
daemon	?
jina	73.08% <ø> (-16.12%)	⬇️

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Impacted Files	Coverage Δ
jina/clients/base.py	71.66% <ø> (-9.17%)	⬇️
jina/executors/indexers/keyvalue.py	98.70% <ø> (ø)
jina/flow/base.py	92.38% <ø> (ø)
jina/helloworld/chatbot/__init__.py	89.28% <ø> (ø)
jina/helloworld/helper.py	90.62% <ø> (ø)
jina/helloworld/multimodal/__init__.py	90.90% <ø> (ø)
jina/logging/logger.py	77.16% <ø> (-5.52%)	⬇️
jina/logging/profile.py	70.07% <ø> (-25.20%)	⬇️
jina/optimizers/__init__.py	0.00% <ø> (-94.40%)	⬇️
jina/optimizers/discovery.py	0.00% <ø> (-80.27%)	⬇️
... and 124 more

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[github-actions]

Latency summary

Current PR yields:

• 😶 index QPS at 910, delta to last 3 avg.: +0%
• 🐢🐢 query QPS at 12, delta to last 3 avg.: -11%

Breakdown

Version	Index QPS	Query QPS
current	910	12
1.0.4	918	13
1.0.3	904	13

Backed by latency-tracking. Further commits will update this comment.