optimize: serializer type meaning

[jsbxyyx]

• I have read the CONTRIBUTING.md guidelines.
• I have registered the PR changes.

Ⅰ. Describe what this PR did

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.77%. Comparing base (0cd141c) to head (f4225f2).
⚠️ Report is 1 commits behind head on 2.x.

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x    #7722      +/-   ##
============================================
- Coverage     61.80%   61.77%   -0.03%     
  Complexity      680      680              
============================================
  Files          1324     1324              
  Lines         50125    50125              
  Branches       5920     5920              
============================================
- Hits          30981    30967      -14     
- Misses        16318    16337      +19     
+ Partials       2826     2821       -5     

Files with missing lines	Coverage Δ
...g/apache/seata/core/serializer/SerializerType.java	75.00% <100.00%> (ø)

... and 8 files with indirect coverage changes

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[xingfudeshi]

Please register this PR in change logs.

[funky-eyes]

LGTM