📖 docs(layout): 优化文档中令人费解的部分

[chenshuai2144]

Checklist

• npm test passes
• tests are included
• documentation is changed or added
• commit message follows commit guidelines

Description of change

• any feature?
• close https://github.com/umijs/umi/ISSUE_URL

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View rendered docs/plugins/plugin-layout.md
View rendered docs/plugins/plugin-layout.zh-CN.md

[codecov]

Codecov Report

Merging #5655 (8249b72) into master (2cf8330) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5655   +/-   ##
=======================================
  Coverage   83.08%   83.08%           
=======================================
  Files         154      154           
  Lines        3441     3441           
  Branches      927      929    +2     
=======================================
  Hits         2859     2859           
  Misses        573      573           
  Partials        9        9           

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