document variable vnode count

[BugenZhao]

Variable vnode count support is implemented during v2.0 to v2.2, which is believed to be stable enough now to be documented and published.

From users' prospective, there's a new session variable named STREAMING_MAX_PARALLELISM which determines the max parallelism to be adopted for future streaming jobs. Consequently, STREAMING_PARALLELISM for a new job cannot exceed this value, and ALTER .. SET PARALLELISM for an existing job cannot exceed this value used to create that job.

The default value is 256. The valid range in 2..=32768. Setting it to a larger value allows a resource-intensive job to use more resources, but this may compromise the performance of range scans on that table or materialized view, if applicable.

The max parallelism for a job cannot be changed once it's created, unless drop and recreate it. All existing jobs prior to this PR will be assigned a default value of 256.