Clarify that originId cannot be a parent of a taskId

[agluszak]

In the description of the build/taskStart method there's a leftover from a previous version of the spec:

Tasks that are spawned by another task should reference the originating task's taskId in their own taskId's parent field. Tasks spawned directly by a request should reference the request's originId parent.

However we previously agreed that "OriginId should not be included in the parents field, there is a separate field for that.". So the sentence

Tasks spawned directly by a request should reference the request's originId parent.

should be replaced with

Tasks spawned directly by a request should have no parents.