Improve the document of learning.Graph

[acezen]

What do these changes do?

Related issue number

Fixes #1274

[codecov-commenter]

Codecov Report

Merging #1277 (6b9fb6e) into main (06e3392) will decrease coverage by 1.22%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1277      +/-   ##
==========================================
- Coverage   73.71%   72.49%   -1.23%     
==========================================
  Files          88       88              
  Lines        9493     9493              
==========================================
- Hits         6998     6882     -116     
- Misses       2495     2611     +116     

Impacted Files	Coverage Δ
python/graphscope/client/session.py	74.95% <ø> (ø)
python/graphscope/tests/unittest/test_java_app.py	55.00% <0.00%> (-45.00%)	⬇️
python/graphscope/analytical/app/java_app.py	24.24% <0.00%> (-44.95%)	⬇️
python/graphscope/framework/app.py	89.44% <0.00%> (-0.51%)	⬇️
python/graphscope/client/rpc.py	85.95% <0.00%> (+0.82%)	⬆️

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[yecol]

Please use tools to check spelling and grammar.

[acezen]

The spell and grammar have been fixed, thanks for the review.