Additional fixes for Solr >= 4.0

Made some minor modifications to be able to use a new solr4 feature (a) and get rid of some warnings in the solr log (b).
a) Added "softCommit" (http://wiki.apache.org/solr/UpdateXmlMessages#A.22commit.22_and_.22optimize.22)
b) Replaced "allowDups", "overwritePending" and "overwriteCommitted" with
"overwrite" (http://wiki.apache.org/solr/UpdateXmlMessages#Optional_attributes_for_.22add.22)
c) Updated phpdocs accordingly