Formats the top passages from content
into a human-readable text snippet.
Parameters:
passages - top-N passages for the field. Note these are sorted in
the order that they appear in the document for convenience.
content - content for the field.
Returns:
formatted highlight. Note that for the
non-expert APIs in PostingsHighlighter that
return String, the toString method on the Object
returned by this method is used to compute the string.