doc: fix broken omprog doc
omprog: ability to execute multiple program instances per action

It can now execute one program instance per worker thread. This is
generally a very good thing the have performance wise. Usually, this
should cause no problems with the invoked program. For that reason,
we have decided to make this the default mode of operation. If not
desired, it can be turned off via the 'forceSingleInstance="on"'
action parameter.
CHANGE OF BEHAVIOUR: previous versions did always execute only one
instance per action, no matter how many workers were active. If
your program has special needs, you need to change your configuration.