GUC: adjust effective_cache_size SQL descriptions

Follow on patch for commit 3e0f1a47.

Reported-by: Peter Eisentraut

Discussion: https://postgr.es/m/369ec766-b947-51bd-4dad-6fb9e026439f@2ndquadrant.com

Backpatch-through: 9.4

src/backend/utils/misc/guc.c

@@ -2987,8 +2987,8 @@ static struct config_int ConfigureNamesInt[] =
 
 	{
 		{"effective_cache_size", PGC_USERSET, QUERY_TUNING_COST,
-			gettext_noop("Sets the planner's assumption about the size of the data cache."),
-			gettext_noop("That is, the size of the cache used for PostgreSQL data files. "
+			gettext_noop("Sets the planner's assumption about the total size of the data caches."),
+			gettext_noop("That is, the total size of the caches (kernel cache and shared buffers) used for PostgreSQL data files. "
 						 "This is measured in disk pages, which are normally 8 kB each."),
 			GUC_UNIT_BLOCKS,
 		},