'makeinfo --help' uses <strong> and <em> for those entries. Replace
that with CSS classes.
* doc/build.scm (html-manual-identifier-index)[build]: Adjust to handle
rewritten forms of <dt> entries.
* doc/build.scm (syntax-highlighted-html)[build][syntax-highlight]:
Handle <dt> forms and replace them.
[highlight-definition, space?]: New procedures.