Stop FVH from throwing away some query boosts

The FVH was throwing away some boosts on queries stopping a number of
ways to boost phrase matches to the top of the list of fragments from
working.

The plain highlighter also doesn't work for this but that is because it
doesn't support the concept of the same term having a different score at
different positions.

Also update documentation claiming that FHV is nicer for weighing terms
found by query combinations.

Closes #4351