Rename pending_list_cleanup_size to gin_pending_list_limit.

Since this parameter is only for GIN index, it's better to
add "gin" to the parameter name for easier understanding.

src/backend/access/gin/ginfast.c

@@ -26,7 +26,7 @@
 #include "utils/rel.h"
 
 /* GUC parameter */
-int			pending_list_cleanup_size = 0;
+int			gin_pending_list_limit = 0;
 
 #define GIN_PAGE_FREESIZE \
 	( BLCKSZ - MAXALIGN(SizeOfPageHeaderData) - MAXALIGN(sizeof(GinPageOpaqueData)) )
@@ -426,7 +426,7 @@ ginHeapTupleFastInsert(GinState *ginstate, GinTupleCollector *collector)
 	 * call it when it can do all the work in a single collection cycle. In
 	 * non-vacuum mode, it shouldn't require maintenance_work_mem, so fire it
 	 * while pending list is still small enough to fit into
-	 * pending_list_cleanup_size.
+	 * gin_pending_list_limit.
 	 *
 	 * ginInsertCleanup() should not be called inside our CRIT_SECTION.
 	 */

src/backend/access/gin/ginutil.c

@@ -526,7 +526,7 @@ ginoptions(PG_FUNCTION_ARGS)
 	int			numoptions;
 	static const relopt_parse_elt tab[] = {
 		{"fastupdate", RELOPT_TYPE_BOOL, offsetof(GinOptions, useFastUpdate)},
-		{"pending_list_cleanup_size", RELOPT_TYPE_INT, offsetof(GinOptions,
+		{"gin_pending_list_limit", RELOPT_TYPE_INT, offsetof(GinOptions,
 																pendingListCleanupSize)}
 	};