Docs: Move parallel_leader_participation GUC description under relevant category

Hi,

It looks like even though the commit e5253fdc4f that added the parallel_leader_participation GUC correctly categorized it as RESOURCES_ASYNCHRONOUS parameter in the code, but in the docs it is kept under irrelevant section i.e. "Query Planning/Other Planner Options". This is reported in the bugs list [1], cc-ed the reporter.

Attaching a small patch that moves the GUC description to the right place. Thoughts?

[1] https://www.postgresql.org/message-id/16972-42d4b0c15aa1d5f5%40postgresql.org

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com