Clarify how step `operationId` and/or `workflowId` should be prefixed if multiple `sourceDescriptions` are defined

[frankkilcommins]

Section Step Object, field operationId

If more than one (non workflowsSpec type) source description is defined within a Workflows Description, then the operationId specified MUST be prefixed with the source name to avoid ambiguity or potential clashes.

How prefixed? Just prepend without separator character? An example would be useful.

Same with field workflowId.

Originally posted by @ralfhandl in #49 (comment)