OAK-6223: Expose socket keep-alive option
OAK-6223: Expose socket keep-alive option

Add documentation since when this option is available.

git-svn-id: 67138be12999c61558c3dd34328380c8e4523e73@1798837 13f79535-47bb-0310-9956-ffa450edef68

davidegiannella/jackrabbit-oak,davidegiannella/jackrabbit-oak,davidegiannella/jackrabbit-oak,davidegiannella/jackrabbit-oak,davidegiannella/jackrabbit-oak