Write specification for new `always` construct, and update docs for `spawn`

[jopperm]

Instead of a single spawn-concept, we propose to introduce separate constructs. A new always-section shall include behavior that is repeatedly executed at the same rate as new instructions are fetched. The existing spawn-keyword marks statements that are allowed to execute concurrently with other instructions in the main pipeline.

[jopperm]

Initial version is done (diff) -- we'll fine-tune the spec as we implement it in the front-end and downstream tools.