Improve documentation for the Dataflow Service options

[damccorm]

Class DataflowPipelineOptions has the ability to set "Dataflow service options".

The description in the code is: 

Service options are set by the user and configure the service. This decouples service side feature
availability from the Apache Beam release cycle.

It would be great to have a link here to the Google cloud documentation where we can find a list of options or perhaps an example on how such an option looks like.

See also:  #14159 (comment)

Imported from Jira BEAM-13630. Original Jira may contain additional context.
Reported by: mcasters.

[pcoet]

.take-issue

[rszper]

Page to link to: https://cloud.google.com/dataflow/docs/reference/service-options

[sereanaseim]

.take-issue

[rszper]

.take-issue