DOC: Specificities section, describe the way to get specificities from an index and a lexical table

Specificities section, describe the way to get specificities from an index and a lexical table.
Also describe what is really done when user creates specificities on a partition from the “Specificities” command. (results seems to be a few different between using the command with ‘word’ and using an index on ‘.*’)

(from redmine: issue id 939, created on 2014/07/15 by Sebastien Jacquot)