intrapara-anchor needs to be more flexible.

For example the doc HTML for `xref-binding->definition-tag` varies
slightly from all the other examples I'd found so far.

Frankly this is a hack attached with masking tape to another hack. It
would be wonderful if Scribble HTML were more "semantic", with e.g. a
clean <div class="defproc"> encapsulating the output from each defproc
form. Even better would be an official Scribble library to extract
this (which could either use such new/improved semantic HTML output, or,
at least confine the ugly hackery to one official place).