Clarify resource utilization of parallel query.

temp_file_limit is a per-process limit, not a per-session limit across
all cooperating parallel processes; change wording accordingly, per a
suggestion from Tom Lane.

Also, document under max_parallel_workers_per_gather the fact that each
process involved in a parallel query may use as many resources as a
separate session.  Caveat emptor.

Per a complaint from Peter Geoghegan.

src/backend/utils/misc/guc.c

@@ -1947,7 +1947,7 @@ static struct config_int ConfigureNamesInt[] =
 
 	{
 		{"temp_file_limit", PGC_SUSET, RESOURCES_DISK,
-			gettext_noop("Limits the total size of all temporary files used by each session."),
+			gettext_noop("Limits the total size of all temporary files used by each process."),
 			gettext_noop("-1 means no limit."),
 			GUC_UNIT_KB
 		},

src/backend/utils/misc/postgresql.conf.sample

@@ -138,7 +138,7 @@
 
 # - Disk -
 
-#temp_file_limit = -1			# limits per-session temp file space
+#temp_file_limit = -1			# limits per-process temp file space
 					# in kB, or -1 for no limit
 
 # - Kernel Resource Usage -