Add description about GSSOpenServer wait event into document.
This commit also updates wait event enum into alphabetical order.
Previously the enum entry for GSSOpenServer was added out-of-order.
Back-patch to v12 where commit
b0b39f72b9 introduced
GSSOpenServer wait event. In v12, the commit doesn't include
the update of wait event enum, not to break ABI.
Author: Fujii Masao
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/
949931aa-4ed4-d867-a7b5-
de9c02b2292b@oss.nttdata.com