Fix comment in pg_get_shmem_allocations_numa()

The comment fixed in this commit described the function as dealing with
database blocks, but in reality it processes shared memory allocations.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/aH4DDhdiG9Gi0rG7@ip-10-97-1-34.eu-west-3.compute.internal
Backpatch-through: 18

src/backend/storage/ipc/shmem.c

@@ -606,16 +606,13 @@ pg_get_shmem_allocations_numa(PG_FUNCTION_ARGS)
 	nodes = palloc(sizeof(Size) * (max_nodes + 1));
 
 	/*
-	 * Different database block sizes (4kB, 8kB, ..., 32kB) can be used, while
+	 * Shared memory allocations can vary in size and may not align with OS
-	 * the OS may have different memory page sizes.
+	 * memory page boundaries, while NUMA queries work on pages.
 	 *
-	 * To correctly map between them, we need to: 1. Determine the OS memory
+	 * To correctly map each allocation to NUMA nodes, we need to: 1.
-	 * page size 2. Calculate how many OS pages are used by all buffer blocks
+	 * Determine the OS memory page size. 2. Align each allocation's start/end
-	 * 3. Calculate how many OS pages are contained within each database
+	 * addresses to page boundaries. 3. Query NUMA node information for all
-	 * block.
+	 * pages spanning the allocation.
-	 *
-	 * This information is needed before calling move_pages() for NUMA memory
-	 * node inquiry.
 	 */
 	os_page_size = pg_get_shmem_pagesize();