Add moduleNameMapper ordering note to the documentation

[emileber]

Summary

Related to #870 (comment)

Just a note for future readers, the moduleNameMapper ordering is important.

I wanted to stub style imported in modules, something like:

// module.js
import Style from '@/path/to/style.scss';
import App from './App';

So I created a style stub file:

// test/unit/stubs/style.js
module.exports = '.style-stub{color:red;}';

After messing around with the following jest.conf.js:

moduleNameMapper: {
    '^@/(.*)$': '<rootDir>/src/$1', // '@' alias
    '^.*\\.scss$': '<rootDir>/test/unit/stubs/style.js',
}

The @ alias rule was resolving before my .scss rule, so the style file was loaded as a normal module and would crash the test.

The solution was to put specific rules first.

moduleNameMapper: {
    '^.*\\.scss$': '<rootDir>/test/unit/stubs/style.js',
    '^@/(.*)$': '<rootDir>/src/$1',
}

[codecov-io]

Codecov Report

Merging #5249 into master will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5249      +/-   ##
==========================================
- Coverage   61.25%   61.18%   -0.08%     
==========================================
  Files         203      202       -1     
  Lines        6786     6771      -15     
  Branches        3        4       +1     
==========================================
- Hits         4157     4143      -14     
+ Misses       2628     2627       -1     
  Partials        1        1

Impacted Files	Coverage Δ
packages/jest-diff/src/index.js	84.09% <0%> (ø)	⬆️
packages/pretty-format/src/index.js	96.45% <0%> (ø)	⬆️
packages/jest-matcher-utils/src/index.js	100% <0%> (ø)	⬆️
packages/jest-snapshot/src/plugins.js	80% <0%> (ø)	⬆️
...ckages/pretty-format/src/plugins/dom_collection.js

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[github-actions]

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