Update fork documentation

[eouthwaite]

Documentation around parallel processes - eg https://github.com/marklogic-community/marklogicworkflow/blob/develop/documentation/TECHINTRO.md#bpmn2-workflow-extensions-on-top-of-cpf - is out of date

[eouthwaite]

See #4

[eouthwaite]

This was also written before parallel gateways were implemented https://github.com/marklogic-community/marklogicworkflow/blob/develop/documentation/WEEDS.md

[eouthwaite]

https://github.com/marklogic-community/marklogicworkflow/blob/develop/documentation/features/feature-4-fork-rendezvous.md