This change ensures that dfn references/links inside sections which follow
the class=domintro convention are output with IDs ending with “-dev” —
and most likely, without any additional ①, ②, etc., suffix.

Otherwise, without this change, dfn references/links inside class=domintro
sections are output with IDs of the form “ref-foo-①”, etc. — just as the
IDs output for such references/links outside of class=domintro sections.

So this change helps make it likely that dfn references/links inside
class=domintro sections end up with stable IDs useful for referencing from
MDN and other places — rather than “ref-foo-②”, etc.  IDs that might change
if some new reference to a term is added to a spec somewhere preceding a
class=domintro section where the term is referenced.

Relates to mdn/browser-compat-data#11088

For any element with an attribute named “subdfn”, this change ensures
the element’s ID remains stable rather than getting “de-duped”; that is,
this change prevents any element with a “subdfn” attribute from being
among the ones that end up with ①, ②, etc., numbered suffixes appended.