syntax-check: Enforce <code> inside <dt> elements

syntax-check: Enforce <code> inside <dt> elements

Commit 61b070cf20b5 cleaned up a number of cases where the <dt>
element was used to document symbols, but the symbol itself was
not inside a <code> element.

To make sure we don't end up having to clean up again a few
months from now, introduce a syntax-check rule that can spot
such mistakes.

All existing exceptions are marked as such, with either file
or line granularity depending on the case.