Unbreak "can't find section" backlink placement.

Sometimes, the zeroth pod block is a paragraph or abbreviated block,
(e.g. a =comment at the start) making adding a backlink normally
generate errors due to the backlink being outside of Pod6-land. So, we
instead just add a new pod block to the front of the list.

While it's certainly possible for this to occur with other blocks in the
list, for the synopses it's very unlikely to occur.