html-has-lang rule seems redundant with lang

[denis-sokolov]

The rule html-has-lang is fully superseded by the lang rule. Would we consider marking the former as deprecated in favor of the latter one?

As I was reviewing the list of rules, I got confused and had to figure out whether I misunderstood something. Having one marked as deprecated would really help. I don’t see any particular reason for html-has-lang to exist. Alternatively, if there is an important case that I miss, we could add explanation to the docs as to the similarity of the two.

Related: #750.