mm: fix kernel-doc warning in page_alloc.c

Fix new kernel-doc warning in mm/page_alloc.c:

  Warning(mm/page_alloc.c:2370): No description found for parameter 'nid'
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -2358,6 +2358,7 @@ EXPORT_SYMBOL(alloc_pages_exact);
 /**
  * alloc_pages_exact_nid - allocate an exact number of physically-contiguous
  *			   pages on a node.
+ * @nid: the preferred node ID where memory should be allocated
  * @size: the number of bytes to allocate
  * @gfp_mask: GFP flags for the allocation
  *