omprog: Add documentation for "forceSingleInstance"

[alorbach]

trafficstars

Needs to be added into the omprog documentation.

The setting was added with this commit: https://github.com/rsyslog/rsyslog/commit/f75395172eac1dfcc63c06ec050739b952eb5cb8#diff-e3cb3014ecfd8b9782b9274c6a7fa343

Seems to be an issue depending on the some user scripts like here: https://github.com/rsyslog/rsyslog/issues/473

[jsiwrk]

This setting must be documented as deprecated, as explained here. I'll add documentation for this flag together with the other undocumented omprog parameters.