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/utils/misc/postgresql.conf.sample

@@ -519,7 +519,7 @@
 #bytea_output = 'hex'			# hex, escape
 #xmlbinary = 'base64'
 #xmloption = 'content'
-#pending_list_cleanup_size = 4MB
+#gin_pending_list_limit = 4MB
 
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