In 2789816 Kevin Hawkins claims that the <desc> of tagdoc for <textLang> is confusing. I agree, and do not understand why the tracker artifact was closed without addressing the problem. (Yes, Kevin's suggested wording isn't right, but that doesn't mean his complaint is invalid.)
My suggestion would be something like:
describes, in a manuscript description, the languages and
writing systems used by the manuscript being described (as
opposed to the languages and writing systems of the
description itself, which are indicated by the
<att>xml:lang</att> attributes of the description).