Fix a couple of wait event descriptions.

Fix a couple of wait event descriptions.

The descriptions for ProcArrayGroupUpdate and XactGroupUpdate claim
that these events mean we are waiting for the group leader "at end
of a parallel operation," but neither pertains to parallel
operations.  This commit reverts these descriptions to their
wording before commit 3048898e73, i.e., "end of a parallel
operation" is changed to "transaction end."

Author: Sameer Kumar
Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/CAGPeHmh6UMrKQHKCmX%2B5vV5TH9P%3DKw9en3k68qEem6J%3DyrZPUA%40mail.gmail.com
Backpatch-through: 13