Synchronize the event names for parallel hash join waits with other
event names, by getting rid of the slashes and dropping "-ing"
suffixes.  Rename ClogGroupUpdate to XactGroupUpdate, to match the
new SLRU name.  Move the ProcSignalBarrier event to the IPC category;
it doesn't belong under IO.

Also a bit more wordsmithing in the wait event documentation tables.

Discussion: https://postgr.es/m/4505.1589640417@sss.pgh.pa.us