[Docs] Document the similarity between html-has-lang and lang

[denis-sokolov]

The simplest way to resolve #777.

[codecov]

Codecov Report

Merging #778 (0123ab1) into master (426d4c2) will decrease coverage by 0.00%.
The diff coverage is n/a.

❗ Current head 0123ab1 differs from pull request most recent head df3c7ad. Consider uploading reports for the commit df3c7ad to get more accurate results

@@            Coverage Diff             @@
##           master     #778      +/-   ##
==========================================
- Coverage   99.24%   99.24%   -0.01%     
==========================================
  Files          98       98              
  Lines        1453     1452       -1     
  Branches      477      477              
==========================================
- Hits         1442     1441       -1     
  Misses         11       11              

Impacted Files	Coverage Δ
src/util/hasAccessibleChild.js	100.00% <0.00%> (ø)
src/rules/interactive-supports-focus.js	97.05% <0.00%> (ø)
__tests__/__util__/ruleOptionsMapperFactory.js	100.00% <0.00%> (ø)

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[jessebeach]

Thank you @denis-sokolov !