Document JbpmSchema API and ant task.

For instance, to drop jBPM schema you can use the two ways.

Programatically:
JbpmSchema.dropSchema()

Through Ant:
<jbpmschema output="${scriptsdir}/jbpm.jpdl.postgresql.sql"
config="hibernate.cfg.postgresql.xml" action="drop" delimiter=";"/>

We need to elaborate how to use those in our docs.